Crude oil tank fabricated from steel plate

ABSTRACT

The present invention provides: a steel for a welded structure to be used for a crude oil tank that exhibits excellent general and local corrosion resistance in crude oil corrosion caused in a steel oil tank and is capable of suppressing the formation of corrosion products (sludge) containing solid sulfur; a method for producing said steel; a crude oil tank; and a method for preventing a crude oil tank against corrosion. The present invention makes it possible to obtain general and local corrosion resistance in a crude oil tank environment and suppress the formation of corrosion products (sludge) containing solid sulfur by using a steel: containing, in mass, 0.001 to 0.2% C, 0.01 to 2.5% Si, 0.1 to 2% Mn, 0.03% or less P, 0.007% or less S, 0.01 to 1.5% Cu, 0.001 to 0.3% Al, 0.001 to 0.01% N as basic components and, further, 0.01 to 0.2% Mo and/or 0.01 to 0.5% W; and preferably satisfying the following expression;
 
Solute Mo+Solute W≧0.005%.

TECHNICAL FIELD

The present invention relates to: a steel for a welded structure to beused for a crude oil tank such as an oil tank of a crude oil carrier oran aboveground or underground crude oil tank, the steel exhibitingexcellent resistance to the corrosion that is caused by crude oil andoccurs in a steel oil tank for transporting or storing crude oil andbeing capable of suppressing the formation of a corrosion product(sludge) containing solid sulfur; a method for producing the steel; acrude oil tank; and a method for protecting the crude oil tank againstcorrosion.

BACKGROUND ART

A steel for a welded structure excellent in strength and weldability isused for a steel oil tank, such as an oil tank of a crude oil carrier oran aboveground or underground crude oil tank, for transporting orstoring crude oil. The problems to be solved in relation to corrosiondamage of a crude oil tank have been: 1) to alleviate corrosion of steelplates, especially to alleviate local corrosion damage in the form ofpitting that progresses at a comparatively high rate; and 2) to reducethe amount of solid sulfur that precipitates on the surfaces of steelplates in a gas phase and causes sludge to form. These problems areoutlined below.

1) Alleviation of Corrosion of Steel Plates

The inside of a crude oil tank is exposed to a corrosive environmentcaused by water, salts and corrosive gas components contained in crudeoil (cf. Recommended Practice of Corrosion Control and Protection inAboveground Oil Storage Tank HPIS G, p. 18 (1989-90), published by theHigh Pressure Institute of Japan, and SR242—Study on Cargo Oil TankCorrosion of Oil Tanker, Outline of Research Activities in Fiscal Year2000 of the Shipbuilding Research Association of Japan). A peculiarcorrosive environment forms especially on the inside of an oil tank of acrude oil carrier because of elements such as volatile components ofcrude oil, contaminating seawater, salts in oil field brine, the marineengine exhaust gas called inert gas that is introduced to the tank forpreventing explosions, and water condensation caused by the temperaturefluctuation between daytime and nighttime. In such an environment, asteel is damaged by general corrosion and local corrosion in the form ofpitting.

As a result, corrosion cavities roughly 10 to 30 mm in diameter form inquantities on the floor plate of an oil tank of a crude oil carrier, andthe corrosion cavities progress at a rate of 2 to 3 mm per year. This isfar greater than the average rate of thickness loss caused by corrosion,0.1 mm per year, which is taken into consideration in the design of ahull. The local corrosion of structural members of a crude oil tank isparticularly detrimental, because when corrosion progresses locally,loads on the corroded portions increase beyond what is expected in thedesign, and large strain and/or plastic deformation occur(s), leading topossible destruction of the whole structure. Thus, countermeasuresagainst local corrosion are indispensable. In addition, it is difficultto predict the location of local corrosion and its rate of progress. Forthese reasons, development of a steel for a welded structure excellentin strength and weldability and, at the same time, having good corrosionresistance especially capable of decreasing the progress rate of localcorrosion has been in demand.

2) Reduction of the Amount of Solid Sulfur That Precipitates on theSurfaces of Steel Plates in a Gas Phase and Causes Sludge to Form

In addition to the corrosion damage mentioned above, a large quantity ofsolid sulfur forms and precipitates on the internal surface of a steeloil tank, especially on the reverse side surface of a steel plate of anupper deck (deck plate). This is because SO₂ and H₂S in a gas phasereact and form solid sulfur, with the iron rust on a corroded steelplate surface acting as a catalyst. The formation of fresh rustresulting from the corrosion of a steel plate and the precipitation ofsolid sulfur take place alternately and, as a result, a multi-layeredcorrosion product composed of iron rust and solid sulfur forms. Since asolid sulfur layer is brittle, the corrosion product composed of ironrust and solid sulfur easily exfoliates, falls off and accumulates assludge at the bottom of an oil tank. The amount of sludge collected froma very large crude oil carrier during a periodical inspection isreported to amount to 300 tons or more, and for that reason, reductionof the amount of sludge composed mainly of solid sulfur has beenrequired from the viewpoint of maintenance.

Corrosion prevention by painting and lining has generally been employedas a technique for protecting a steel material against corrosion andsimultaneously decreasing sludge composed mainly of solid sulfur, andcorrosion prevention by spraying zinc and/or aluminum has also beenproposed (cf. Recommended Practice of Corrosion Control and Protectionin Aboveground Oil Storage Tank HPIS G, p. 18 (1989-90), published bythe High Pressure Institute of Japan). However, in addition to theeconomical problems of the time and costs involved in re-painting thereverse side of all the deck plates of a very large crude oil carrier,there has also been a technical problem in that protection by paintingand/or lining also requires periodical inspections and repair, becausecorrosion inevitably progresses as a result of microscopic defectscaused during the application of protective layers and age-relateddegradation.

Despite the above, no technology to suppress the precipitation of solidsulfur on a steel plate surface by improving the corrosion resistance ofthe steel plate itself in a crude oil tank environment has beendisclosed. In this situation, in the field of a steel for a weldedstructure such as an oil tank, development of a steel for a weldedstructure excellent in corrosion resistance and capable of suppressingthe formation of sludge mainly composed of solid sulfur has been indemand from the viewpoints of enhancing the reliability and extendingthe service life of the structure.

Here, an overview is given below regarding technologies so far proposedto solve the problems 1) and 2) above, peripheral technologies proposedin relation thereto and problems involved in the proposed technologies.

1) Measures to Alleviate Corrosion of Steel Plates and Problems ofConventional Technologies

Technologies so far proposed to alleviate corrosion, especially localcorrosion, of a steel plate occurring on the inside of a crude oil tankare described below. Ordinary steels for welded structures havegenerally been used without protective coating for a crude oil tank,either that of a crude oil carrier or that constructed aboveground orunderground. Painting has conventionally been the most commonly employedcorrosion prevention method, and protective painting with an epoxy resinand/or zinc rich primer, heavy-duty coating with an epoxy resin mixedwith glass flakes and the like have been proposed for this purpose.Besides these, hot dip galvanized steels with paint coating have beenused for handrails and piping of an oil carrier in view of its excellentcorrosion resistance in an environment where the steels are alternatelyexposed to seawater and crude oil. In addition to the above, thefollowing technologies have been proposed to provide corrosion-resistantsteels having better corrosion resistance than ordinary steels do andbeing suitable for use in the interior of a crude oil tank.

Japanese Unexamined Patent Publication No. S50-158515 proposes aCu—Cr—Mo—Sb steel as a steel for an oil loading pipe on the basis thatthe steel exhibits excellent corrosion resistance in an environmentwhere a steel, such as an oil loading pipe, is exposed to crude oil andseawater alternately or simultaneously. The corrosion-resistant steeldisclosed in the publication contains 0.2 to 0.5% Cr as a main componentand, in addition, 0.1 to 0.5% Cu, 0.02 to 0.5% Mo, and 0.01 to 0.1% Sb.

Japanese Unexamined Patent Publication No. 2000-17381 proposes a Cu—Mgsteel as a corrosion-resistant steel for shipbuilding on the basis thatthe steel exhibits excellent corrosion resistance in an environmentwhere a steel is used for a hull outer plate, a ballast tank, an oiltank (crude oil tank) of a crude oil carrier, or a cargo hold of anore/coal carrier. The corrosion-resistant steel disclosed in thepublication contains 0.01 to 2.0% Cu and 0.0002 to 0.0150% Mg as maincomponents and, in addition, 0.01 to 0.25% C, 0.05 to 0.50% Si, 0.05 to2.0% Mn, 0.10% or less P, 0.001 to 0.10% S, and 0.005 to 0.10% Al.

Japanese Unexamined Patent Publication No. 2001-107179 proposes ahigh-P—Cu—Ni—Cr-high-Al steel as a corrosion-resistant steel for an oilloading tank on the basis that the steel exhibits excellent corrosionresistance at the reverse side of the deck plate of an oil loading tankand low welding crack sensitivity. The corrosion-resistant steeldisclosed in the publication contains 0.04 to 0.1% P, 0.005% or less S,0.1 to 0.4% Cu, 0.05 to 0.4% Ni, 0.3 to 4% Cr and 0.2 to 0.8% Al as maincomponents and, in addition, 0.12% or less C, 1.5% or less Si and 0.2 to3% Mn, and satisfies the expression Pcm≦0.22, where Pcm=[% C]+[%Si]/30+[% Mn]/20+[% Cu]/20+[% Ni]/60+[% Cr]/20+[% Mo]/15+[% V]/10+5[%B].

Japanese Unexamined Patent Publication No. 2001-107180 proposes alow-P—Cu—Ni—Cr-high-Al steel as a corrosion-resistant steel for an oilloading tank on the basis that the steel exhibits excellent corrosionresistance at the reverse side of the deck plate of an oil loading tank,as well as being excellent in a balance between mechanical propertiesand weldability at large-heat-input welding exceeding 100 kJ. Thecorrosion-resistant steel disclosed in the publication contains 0.035%or less P, 0.005% or less S, 0.1 to 0.4% Cu, 0.05 to 0.4% Ni, 0.3 to 4%Cr and 0.2 to 0.8% Al as main components and, in addition, 0.12% or lessC, 1.5% or less Si and 0.2 to 3% Mn, and satisfies the expressionPcm≦0.22, where Pcm=[% C]+[% Si]/30+[% Mn]/20+[% Cu]/20+[% Ni]/60+[%Cr]/20+[% Mo]/15+[% V]/10+5[% B].

Japanese Unexamined Patent Publication No. 2002-12940 proposes a Cucontained steel, a Cr contained steel and an Ni contained steel ascorrosion-resistant steels for oil loading tanks and methods forproducing the same on the basis that each of the steels exhibits:excellent corrosion resistance, more specifically, such good durabilityas to minimize the progress of rust under a primer coating film and thusto extend the service life of the coating film after the application ofthe primer coating in a corrosive atmosphere at the upper part of an oilloading tank, i.e. in an acid-dew-point corrosive environment caused bycorrosive components included in the engine exhaust gas that isintroduced into an oil loading tank; and the feature of excellentweldability. Each of the corrosion-resistant steels disclosed in thepublication: is used on condition that primer coating is applied;contains one or more of 0.1 to 1.4% Cu, 0.2 to 4% Cr and 0.05 to 0.7% Nias basic component(s) and, in addition, 0.16% or less C, 1.5% or lessSi, 3.0% or less Mn, 0.035% or less P and 0.01% or less S; and satisfiesthe expression Pcm≦0.22, where Pcm=[% C]+[% Si]/30+[% Mn]/20+[%Cu]/20+[% Ni]/60+[% Cr]/20+[% Mo]/15+[% V]/10+5[% B].

Japanese Unexamined Patent Publication No. 2003-105467 proposes a Cu—Nisteel as a corrosion-resistant steel for an oil loading tank excellentin corrosion resistance at a weld on the basis that the steel exhibitsexcellent corrosion resistance both at base material after applicationof primer coating and at a weld to which primer coating is not appliedand makes it possible to use an existing welding wire for a carbonsteel. The corrosion-resistant steel disclosed in the publication: isused on condition that primer coating is applied; contains 0.15 to 1.4%Cu as a basic component and, in addition, 0.16% or less C, 1.5% or lessSi, 2.0% or less Mn, 0.05% or less P and 0.01% or less S; and satisfiesthe expression Pcm≦0.24, wherePcm=C+Si/30+Mn/20+Cr/20+Cu/20+Ni/60+Mo/15+V/10+5B.

Japanese Unexamined Patent Publication No. 2001-214236 proposes a Cucontained steel, a Cr contained steel, an Mo contained steel, an Nicontained steel, an Sb contained steel, and an Sn contained steel ascorrosion-resistant steels for crude oil or heavy oil storage tanks onthe basis that each of the steels exhibits excellent corrosionresistance when it is used for a crude oil carrier, an oil tank or thelike for storing a liquid fuel or a raw fuel such as crude oil or heavyoil. Each of the corrosion-resistant steels disclosed in the publicationcontains one or more of 0.01 to 2.0% Cu, 0.01 to 7.0% Ni, 0.01 to 10.0%Cr, 0.01 to 4.0% Mo, 0.01 to 0.3% Sb and 0.01 to 0.3% Sn as basiccomponent(s) and, in addition, 0.003 to 0.30% C, 2.0% or less Si, 2.0%or less Mn, 0.10% or less Al, 0.050% or less P and 0.050% or less S.

Japanese Unexamined Patent Publication No. 2002-173736 proposes aCu—Ni—Cr steel as a corrosion-resistant steel for a tank fortransporting or storing crude oil on the basis that the steel exhibitsexcellent corrosion resistance. The corrosion-resistant steel disclosedin the publication contains 0.5 to 1.5% Cu, 0.5 to 3.0% Ni and 0.5 to2.04 Cr as basic components and, in addition, 0.001 to 0.20% C, 0.10 to0.40% Si, 0.50 to 2.0% Mn, 0.020% or less P, 0.010% or less S and 0.01to 0.10% Al.

Japanese Unexamined Patent Publication No. 2003-82435 proposes an Nicontained steel and a Cu—Ni steel as steel materials for cargo oil tankson the basis that each of the steels exhibits excellent corrosionresistance, more specifically, excellent resistance to general corrosionin an environment containing inert gas where wet and dry are repeatedalternately. Each of the corrosion-resistant steels disclosed in thepublication contains 0.05 to 3% Ni as a basic component and, inaddition, 0.01 to 0.3% C, 0.02 to 1% Si, 0.05 to 2% Mn, 0.05% or less P,0.01% or less S and, as required, one or more of Mo, Cu, W, Ca, Ti, Nb,V, B, Sb, and Sn.

In addition to the above, the following technologies have been proposedregarding corrosion resistant steels for a ballast tank of a marinevessel, although the steels are not for crude oil tank use.

Japanese Examined Patent Publication No. 549-27709 proposes a Cu—W steeland a Cu—W—Mo steel as corrosion-resistant low-alloy steels on the basisthat each of the steels exhibits excellent corrosion resistance whenused for a ballast tank. Each of the corrosion-resistant steelsdisclosed in the publication contains 0.15 to 0.50% Cu and 0.05 to 0.5%W basic components and, in addition, 0.2% or less C, 1.0% or less Si,1.5% or less Mn and 0.1% or less P and, as required, 0.05 to 1.0% Mo.

Japanese Unexamined Patent Publication No. S48-509217 proposes, inpatent document 11, a Cu—W steel and a Cu—W—Mo steel ascorrosion-resistant low-alloy steels on the basis that each of thesteels exhibits excellent corrosion resistance when used for a ballasttank. Each of the corrosion-resistant steels disclosed in thepublication contains 0.15 to 0.50% Cu and 0.01 to less than 0.05% W asbasic components and, in addition, 0.2% or less C, 1.0% or less Si, 1.5%or less Mn and 0.1% or less P and, as required, 0.05 to 1.0% Mo.

Japanese Unexamined Patent Publication No. S48-50922 proposes a steelcontaining Cu, W and one or more of Ge, Sn, Pb, As, Sb, Bi, Te and Be asa corrosion-resistant low-alloy steel on the basis that the steelexhibits excellent corrosion resistance, more specifically excellentresistance to local corrosion in a ballast tank. The corrosion-resistantsteel disclosed in the publication contains 0.15 to 0.50% Cu, 0.05 to0.5% W and one or more of Ge, Sn, Pb, As, Sb, Bi, Te and Be by a totalof 0.01 to 0.2% as basic components and, in addition, 0.2% or less C,1.0% or less Si, 1.5% or less Mn and 0.1% or less P and, as required,0.01 to 1.0% Mo.

Japanese unexamined Patent Publication No. 549-3808 proposes a Cu—Mosteel as a corrosion-resistant low-alloy steel on the basis that thesteel exhibits excellent corrosion resistance in a ballast tank, highstrength and good weldability. The corrosion-resistant steel disclosedin the publication contains 0.05 to 0.5% Cu and 0.01 to 1% Mo as basiccomponents and, in addition, 0.2% or less C, 1.0% or less Si, 0.3 to3.0% Mn and 0.1% or less P.

Japanese unexamined Patent Publication No. S49-52117 proposes a Cr—Alsteel as a seawater corrosion-resistant low-alloy steel on the basisthat the steel is excellent in corrosion resistance in seawater, morespecifically in resistance to pitting corrosion and crevice corrosion,which are likely to occur in quantity to a steel containing alloyingelements. The corrosion-resistant steel disclosed in the publicationcontains 1 to 6% Cr and 0.1 to 8% Al as basic components and, inaddition, 0.08% or less C, 0.75% or less Si, 1% or less Mn, 0.09% orless P and 0.09% or less S.

Japanese Unexamined Patent Publication No. H7-310141 proposes a Cr—Tisteel as a seawater corrosion-resistant steel for use in ahigh-temperature and high-humidity environment and a method forproducing the same on the basis that the steel exhibits excellentresistance to seawater corrosion in a high-temperature and high-humidityenvironment of a marine vessel, namely in a ballast tank or in aseawater pipe and excellent toughness at a heat-affected zone (HAZ). Thecorrosion-resistant steel disclosed in the publication contains 0.50 to3.50% Cr as a basic component and, in addition, 0.1% or less C, 0.50% orless Si, 1.50% or less Mn and 0.005 to 0.050% Al.

Japanese Unexamined Patent Publication No. H8-246048 proposes a Crcontained steel in a method for producing a seawater corrosion-resistantsteel excellent in toughness of a HAZ for use in a high-temperature andhigh-humidity environment on the basis that the steel exhibits excellentresistance to seawater corrosion in a high-temperature and high-humidityenvironment of a marine vessel, namely in a ballast tank or a seawaterpipe. The corrosion-resistant steel disclosed in the publicationcontains 1.0 to 3.0% Cr and 0.005 to 0.03% Ti as basic components and,in addition, 0.1% or less C, 0.10 to 0.80% Si, 1.50% or less Mn and0.005 to 0.050% Al.

Here, problems of the conventional technologies described above areexplained.

The problems arising when corrosion is mitigated by means of corrosionprevention coating such as primer coating, heavy-duty coating or metalspraying have been that: the application work entails substantial costs;and, in addition, corrosion develops to a extent comparable to a case ofbare use in 5 to 10 years of normal use at the longest, because localcorrosion inevitably occurs and propagates from microscopic defects inprotective coating layers caused during the application work and otherdefects resulting from age-related degradation. Another problem has beenthat periodical inspections and repair are indispensable and maintenancecosts are involved as a consequence. Yet another problem has been that,with regard to local corrosion at the floor plate of an oil tank, therate of progress of local corrosion occurring after protective coatinglayers have been degraded is substantially the same as that occurring inbare use.

The problems of the steel for an oil loading pipe disclosed in JapaneseUnexamined Patent Publication No. S50-158515 have been that: since itcontains Cr, which is detrimental to corrosion resistance in a crude oiltank environment, in excess of 0.1%, the rate of progress of localcorrosion at the floor plate of an oil tank is not reduced and the costeffect of corrosion resistance is insufficient to justify the totaladdition amount of the alloying elements; and the weldability of thesteel is poor in comparison with an ordinary steel because it containsCr.

The problems of the corrosion-resistant steel for shipbuilding disclosedin Japanese Unexamined Patent Publication No. 2000-17381 have been that:since it contains Mg as an indispensable element, the production of thesteel is unstable; and, according to the studies by the presentinventors, the rate of progress of local corrosion at the floor plate ofan oil tank is not reduced by the use of a Cu—Mg steel and the costeffect of corrosion resistance is insufficient to justify the totaladdition amount of the alloying elements.

The problems of the corrosion-resistant steel for an oil loading tank (ahigh-P—Cu—Ni—Cr-high-Al steel) disclosed in Japanese Unexamined PatentPublication No. 2001-107179 have been that: since it contains Cr, whichis detrimental to corrosion resistance in an environment of a crude oiltank floor plate, by 0.3 to 4% in excess of 0.1%, the rate of progressof local corrosion at the floor plate of an oil tank is not reduced andthe cost effect of corrosion resistance is insufficient to justify thetotal addition amount of the alloying elements; and the weldability ofthe steel is poor in comparison with an ordinary steel because itcontains Cr.

The problems of the corrosion-resistant steel for an oil loading tank (alow-P—Cu—Ni—Cr-high-Al steel) disclosed in Japanese unexamined PatentPublication No. 2001-107180 have been that: since it contains Cr, whichis detrimental to corrosion resistance in an environment of a crude oiltank floor plate, by 0.3 to 4% in excess of 0.1%, the rate of progressof local corrosion at the floor plate of an oil tank is not reduced andthe cost effect of corrosion resistance is insufficient to justify thetotal addition amount of the alloying elements; the weldability of thesteel is poor in comparison with an ordinary steel because it containsCr; and, although the publication maintains that the steel afterapplication of a primer coating suppresses corrosion under a coatingfilm in a gas phase to which the reverse side of a deck plate or thelike of an oil tank is exposed, since the steel contains comparativelylarge amounts of Cr and Al, the rate of corrosion propagating in thethickness direction from defects in a coating film is not reduceddespite the width of blisters occurring from defects in the coating filmbeing reduced.

The problem of the corrosion-resistant steels (Cu—Ni steels) for oilloading tanks disclosed in Japanese Unexamined Patent Publication Nos.2002-12940 and 2003-105467 has been that, though the publicationsmaintain that Cu and Ni are effective in enhancing corrosion resistance,more specifically resistance to corrosion under a coating film, and Mois detrimental to corrosion resistance but is effective for enhancingstrength, since any of the Cu—Ni—Mo steels proposed ascorrosion-resistant steels in the example contains Mo in excess of theupper limit (0.2%) of the present invention, the effect of suppressingthe progress of local corrosion at the floor plate of a crude oil tankis not achieved.

The problems of the corrosion-resistant steels (a Cu contained steel, aCr contained steel, an Mo contained steel, an Ni contained steel, anSb-contained steel and an Sn-contained steel) for crude oil or heavy oilstorage tanks disclosed in Japanese Unexamined Patent Publication No.2001-214236 have been that: large amounts of alloying elements must beadded in order to obtain excellent corrosion resistance as the exampleshows that it is indispensable to add one or more of 0.22 to 1.2% Cu,0.3 to 5.6% Cr, 0.5 to 6.2% Ni, 0.25 to 7.56% Mo, 0.07 to 0.25% Sb and0.07 to 1.5% Sn; and thus the economical efficiency and weldability ofthe proposed steels are poor.

The problems of the corrosion-resistant steel for a tank fortransporting or storing crude oil (a Cu—Ni—Cr steel) disclosed inJapanese Unexamined Patent Publication No. 2002-173736 have been that:the steel contains 0.5 to 1.5% Cu, 0.5 to 3.0% Ni and 0.5 to 2.0% Cr asbasic components, thus large amounts of alloying elements must be addedfor the effect to appear; thus the economical efficiency and weldabilityof the proposed steels are poor; and further, since the steel containsCr, which is detrimental to corrosion resistance in an environment of acrude oil tank floor plate, in excess of 0.1%, the rate of progress oflocal corrosion at the floor plate of an oil tank is not reduced and thecost effect of corrosion resistance is insufficient to justify the totaladdition amount of the alloying elements.

With regard to the steels for cargo oil tanks (Ni contained steels andCu—Ni steels) disclosed in Japanese Unexamined Patent Publication No.2003-82435, steel components are studied which decrease the progress oflocal corrosion in an experimental corrosive environment simulating notthat at the floor plate of an oil tank, but at the reverse side of adeck plate. Table 4 of the publication lists the following as the steelsthat contain Cu, Ni and Mo as basic components but not Cr: sample nos.B4 (0.43% Cu-0.18% Ni-0.26% Mo), B6 (0.33% Cu-0.31% Ni-0.35% Mo), B13(0.38% Cu-0.12% Ni-0.44% Mo), B15 (0-35% Cu-0.28% Ni-0.31% Mo), B19(0.59% Cu-0.16% Ni-0.22% Mo) and B20 (0.59% Cu-0.44% Ni-0.22% Mo). Theproblems of the steels have been that: all of these steels requiresrelatively large addition amounts of alloying components even thoughonly the basic components are taken into consideration and results inunfavorable costs and weldability; and further, in order to realizeexcellent corrosion resistance in an environment of a crude oil tankfloor plate, it is necessary to use an Ni-contained steel or a Cu—Nisteel, control the number of inclusions larger than 30 μm in grain sizeto less than 30/cm², and control the pearlite ratio Ap in themetallographic structure and the carbon content in the steel so as tosatisfy the expression Ap/C≦130.

Next, the problems of the corrosion-resistant steels proposed for theuse in the ballast tank of a marine vessel are explained.

The problems of the corrosion-resistant low-alloy steels (a Cu—W steeland a Cu—W—Mo steel) disclosed in Japanese Examined Patent PublicationNo. S49-27709 have been that: since the steels do not contain Alaccording to the chemical compositions of the invention steels shown inTable 1 of the examples described in patent document 10, resistance tolocal corrosion is not secured in the case of the floor plate of a crudeoil tank; and further the proposed steel, which is not Al-killed steels,is hardly applicable to the latest shipbuilding use from the viewpointsof the cleanliness of the steels and the toughness of welds.

The problems of the corrosion-resistant low-alloy steels (a Cu—W steeland a Cu—W—Mo steel) disclosed in Japanese Unexamined Patent PublicationNo. S48-50921 have been that: since the steels do not contain Alaccording to the chemical compositions of the invention steels shown inTable 1 of the examples described in the patent, resistance to localcorrosion is not secured in the case of the floor plate of a crude oiltank; and further the proposed steel, which is obviously not Al-killedsteels, is hardly applicable to the latest shipbuilding use from theviewpoints of the cleanliness of the steels and the toughness of welds.

The problems of the corrosion-resistant low-alloy steel disclosed inJapanese Unexamined Patent Publication No. S48-50922 have been that:since the steel contains 0.15 to 0.50% Cu, 0.05 to 0.5% W and furtherone or more of Ge, Sn, Pb, As, Sb, Bi, Te and Be by 0.01 to 0.2%, theproposed steel is markedly poor in hot workability; since the steel doesnot contain Al according to the chemical compositions shown in Table 1of the patent, local corrosion resistance is not secured in the case ofa floor plate of a crude oil tank; and further the proposed steel, whichis obviously not an Al-killed steel, is hardly applicable to the latestshipbuilding use from the viewpoints of the cleanliness of the steel andthe toughness of a weld.

The problems of the Cu—Mo steel proposed in Japanese Unexamined PatentPublication No. 549-3808 as a corrosion-resistant low-alloy steel forballast tank use is that: since the steel is obviously required tocontain not less than 0.008% S in order to obtain desired corrosionresistance in a ballast tank environment according to the chemicalcomposition of the proposed steel shown in the examples described in thepatent, the proposed steel cannot secure local corrosion resistancecomparable with that of a steel according to the present invention inthe case of a crude oil tank floor plate; since the steel does notcontain Al, local corrosion resistance is not secured in the case of afloor plate of a crude oil tank; and further the proposed steel, whichis obviously not an Al-killed steel, is hardly applicable to the latestshipbuilding use from the viewpoints of the cleanliness of the steel andthe toughness of a weld.

The problem of the corrosion-resistant steels disclosed in JapaneseUnexamined Patent Publication Nos. S49-52117, H7-310141 and H8-246048has been that each of the steels contains not less than 0.5% Cr as abasic component and cannot secure local corrosion resistance in the caseof the floor plate of a crude oil tank.

Other than the conventional technologies mentioned above, sometechnologies regarding low-alloy corrosion-resistant steels for otherapplications have been disclosed. Some comments are given thereonhereafter.

Automobile undercarriage members suffer wet corrosion involving chlorideions with deicing salt attaching thereto. With regard to low-alloysteels for automobile undercarriage members excellent in pittingcorrosion resistance that cope with such corrosion problem, there are,for instance: the technology characterized by adding Cu, Ni, Ti and P toa steel and, by so doing, forming a protective film composed ofphosphate on the surface of the steel (such as the one disclosed inJapanese Unexamined Patent Publication No. S62-243738); and thetechnology characterized by adding P and/or Cu to a steel and, by sodoing, making the formed rust layer amorphous and dense so as to enhancethe protective capability of the rust layer (such as the one disclosedin Japanese Unexamined Patent Publication No. H2-22416). In addition,many steelmakers have developed and commercialized seawater-resistantlow-alloy steels having improved seawater resistance (cf.“Corrosion-resistant Low-alloy Steel” by Iwao Matsushima, p. 117,published from Chijin Shokan in 1995).

In the case of those steels for automobile undercarriage parts excellentin pitting corrosion resistance and other weatherproof steels, althoughit is true that a protective dense rust layer forms on the surface evenwhen such a steel is used in a salt damage environment, such excellentpitting corrosion resistance is obtained only in an environment wherewet and dry are repeated properly and resultantly a protective denserust layer forms spontaneously and not in an environment where the steelsurface is always wet. Thus, such excellent pitting corrosion resistanceis not obtained in an environment where the time of wetting is long orthe steel surface is always wet. On the other hand, in the case of theseawater-resistant low-alloy steels mentioned above, though they oftenexhibit better performance than ordinary steels regarding the kind ofcorrosion resistance to be evaluated in terms of an average thicknessloss rate, they are not viewed as distinctly superior to ordinary steelsregarding a local corrosion rate of progress (cf. “Corrosion-resistantLow-alloy Steel” by Iwao Matsushima, p. 112, published from ChijinShokan in 1995).

As has hitherto been explained, in the application of a steel to awelded structure such as a crude oil tank, development of a low-alloysteel having a low local corrosion progress rate even though generalcorrosion may occur has been looked for from the viewpoints of enhancingthe reliability of a structure and extending the service life. As forthe technologies for decreasing the progress of local corrosion at thefloor plate of a crude oil tank, merely the methods of applying aprotective lining to the floor plate have been proposed. There have beena number of proposals regarding corrosion-resistant steels to mitigatethe corrosion occurring in the environment of a ballast tank, which issimilar to the environment of a crude oil tank intended in the presentinvention, or in the environment at the reverse side of a deck plate ofa crude oil tank. However, there has been only one proposal regarding acorrosion-resistant steel having a low local corrosion progress rate atthe floor plate of a crude oil tank, which is the invention disclosed inthe Japanese Unexamined Patent Publication No. 2003-82435 mentionedearlier.

2) Measures to Reduce the Amount of Solid Sulfur That Precipitates onthe Surfaces of Steel Plates in a Gas Phase and Causes Sludge to Formand Problems of Conventional Technologies

Corrosion prevention by painting and lining has commonly been employedas a technique to protect steel from corrosion and, at the same time,reduce sludge composed mainly of solid sulfur. Corrosion prevention byspraying zinc and/or aluminum has also been proposed (cf. RecommendedPractice of Corrosion Control and Protection in Aboveground Oil StorageTank HPIS G, p. 18 (1989-90), of the High Pressure Institute of Japan).However, like in the case of corrosion reduction measures, the problemsof the technologies have been that: the application work entailseconomic costs; and, in addition, since corrosion inevitably progressesas a result of microscopic defects in protective layers caused duringthe application work and age-related degradation, periodical inspectionsand repair are indispensable and the service life is limited to 5 to 10years, even when painting and lining are applied.

Despite the above problems, there has been disclosed no technology todecrease the precipitation of solid sulfur on a steel surface byimproving the corrosion resistance of steel itself in a crude oil tankenvironment. In such a situation, in the application of a steel to awelded structure such as an oil tank, development of a steel for awelded structure excellent in corrosion resistance and capable ofdecreasing the formation of sludge mainly composed of solid sulfur hasbeen looked for from the viewpoints of enhancing the reliability of thestructure and extending the service life.

DISCLOSURE OF THE INVENTION

The object of the present invention, which has been established to solvethe above problems, is to provide: a steel for a welded structure to beused for a crude oil tank, the steel exhibiting excellent localcorrosion resistance in an environment of the floor plate of a crude oiltank and decreasing the rate of formation of a corrosion productcontaining solid sulfur in a gas phase at the reverse side of the upperdeck plate of a crude oil tank; a method for producing the steel; acrude oil tank; and a method for protecting the crude oil tank againstcorrosion.

The present inventors, in an attempt to solve the aforementionedproblems, investigated the influences of chemical components,metallographic structures and production methods on the behavior ofprogress of local corrosion at the floor plate of a crude oil tank andthe behavior of precipitation of solid sulfur at the reverse side of anupper deck plate, and as a result made the following discoveries:

[1] Means to Suppress the Progress of Local Corrosion at the Floor Plateof a Crude Oil Tank

A great amount of rock salt brine is contained in crude oil and itseparates from the oil and remains on the floor plate of a crude oiltank. The present inventors found, first, that the concentration of suchrock salt brine, which varied in accordance with the oil field and thedepth of an oil well from which the crude oil came, was as high asroughly 1 to 60 mass % in terms of an NaCl-reduced concentration. Theyalso found out that, when a steel plate was exposed to suchhigh-concentration brine, or a high-concentration aqueous solution ofhalogen, the following occurred: the condition at the surface of thesteel plate became uneven by sediment of corrosion products, sludge, ashand the like; the sites where the base steel dissolved selectively werequickly formed and fixed; and local corrosion developed from thesesites. Further, on the basis of the above discoveries, the presentinventors proposed the following corrosion mechanism: the pH bufferingcapacity of the high-concentration brine was so small that the value ofpH rapidly fell to 2 or lower at the sites where the base steeldissolved selectively as a consequence to the hydrolysis of dissolvedions of iron and alloying elements, and local corrosion developed fromthose sites in a catalytically accelerated manner.

Further, the present inventors studied the influences of Cu and Mo onthe rate of progress of local corrosion using Fe—Cu—Mo steels, whichcontained various addition amounts of Cu (0.1 to 0.5 mass %) and Mo(0.025 to 0.075 mass %), produced in a laboratory and, as a result, madethe findings set out below.

FIG. 1 shows the influence of an addition amount of Mo on the rate ofprogress of local corrosion of Fe—Cu—Mo steels. The present inventorsfound from the figure that the rate of progress of local corrosion fellto minimum when the Mo content was roughly 0.05 mass % and the localcorrosion reduction effect of Mo decreased when its content was 0.1 mass% or more. As a consequence, it became clear that the most desirable Moaddition amount was in the range from 0.03 to 0.07%.

FIG. 2 shows the influence of an addition amount of Cu on the rate ofprogress of local corrosion of Fe—Cu—Mo steels. The present inventorsfound from the figure that the remarkable effect of combined Cu—Moaddition on suppressing the rate of progress of local corrosion wasobserved when the Cu amount was not less than 0.1 mass %, and the effectbecame substantially saturated when the Cu amount reached 0.3%.

FIGS. 3( a) and (b) show the influences of the contents of P and S,respectively, on the rate of progress of local corrosion of 0.3%Cu-0.05% Mo steels. Either of P and S, which were impurity elements,tended to accelerate the progress of local corrosion: the rate ofprogress of local corrosion increased significantly when the P contentexceeded 0.03% or the S content exceeded 0.02%. It was also clear thatthe detrimental effects of these elements could be minimized when the Pcontent was not more than 0.010% or the S content was not more than0.0070%.

FIG. 4 shows the influence of an addition amount of Al on the rate ofprogress of local corrosion of Low-P-low-S—Cu—Mo steels. The rate ofprogress of local corrosion followed a downward convex curve, and itincreased when the Al content exceeded 0.3%. Further, it was clear thatlocal corrosion resistance was enhanced yet more when the Al content wascontrolled to 0.01 to 0.1%.

The above findings can be summarized as follows:

{circle around (1)} When 0.01 to 0.1 mass % Mo is added to a steelcontaining not less than 0.1 mass % Cu, the rate of progress of localcorrosion is remarkably decreased to not more than ⅕ that of an ordinarysteel.

{circle around (2)} When more than 0.1 mass % Mo is added to a steelcontaining not less than 0.1 mass % Cu, the effect of Mo on suppressingthe rate of progress of local corrosion decreases.

{circle around (3)} In the case of a steel containing not less than 0.1mass % Cu, the most suitable addition amount of Mo is in the range from0.03 to 0.07 mass %.

{circle around (4)} An excessive addition of either P or S acceleratesthe rate of progress of local corrosion, and excellent local corrosionresistance is obtained by setting the upper limits of the contents of Pand S.

{circle around (5)} when the addition amount of Al is controlled to 0.01to 0.1%, local corrosion resistance is enhanced yet further.

{circle around (6)} Cr is a harmful element that accelerates theprogress of local corrosion significantly, and it is desirable tocontrol its content to 0.01% or less.

A feature of the present invention is to decrease the rate of progressof local corrosion at corroded portions after the formation of localcorrosion, on the basis of the above and other findings by the presentinventors.

Further, the present inventors further investigated and, as a result,made the findings set out below.

Specifically, the following results were obtained, on the basis of thechemical composition of a common steel for a welded structure,substantially not adding Cr, by adding specific amount(s) of Mo and/or Win combination with Cu, limiting the addition amounts of P and S, whichare impurity elements, and adding Al:

1) when contents of P, S, and Al are controlled to respectively definedranges, the rate of progress of the local corrosion in the environmentin question decreases remarkably with the smaller addition amounts ofalloying elements of Cu, Mo and W; and

2) according to the results of detailed studies on the relationshipbetween the state of Mo and W in a steel and corrosion resistance, whenMo and W exist in a steel in the state of solid solution, their effectsin enhancing corrosion resistance are further increased.

[2] Means to Reduce Solid Sulfur That Precipitates From a Gas Phase onthe Reverse Side of the Upper Deck Plate of a Crude Oil Tank and CausesSludge to Form

As a result of extensive study of the precipitation behavior of solidsulfur from a gas phase on the surface of a steel plate used as theupper deck plate of a crude oil tank, the present inventors made thefollowing findings: {circle around (1)} solid sulfur precipitates as aresult of a reaction of hydrogen sulfide and oxygen in a gas phase in acrude oil tank with iron rust on the surface working as a catalyst;{circle around (2)} the precipitation rate of the solid sulfur dependson the temperature, the concentrations of hydrogen sulfide and oxygen inthe gas phase and, further, on alloying elements included in the ironrust in minimal quantities; {circle around (3)} when both Cu and Mo areincluded in the iron rust, the precipitation rate of the solid sulfurdecreases; and {circle around (4)} when both Cu and Mo are included inthe iron rust, the rate of progress of general corrosion in theenvironment in question also decreases. On the basis of the abovefindings, the present inventors discovered that it was possible toenhance corrosion resistance, or resistance to general corrosion, in theenvironment in question by not adding Cr, adding Cu and Mo incombination by respectively defined amounts and limiting the additionamounts of P and S, which are impurity elements, on the basis of thechemical composition of a common steel for a welded structure.

The gist of the present invention, which has been established-basedmainly on the above findings, is as follows:

-   -   (1) A steel for a crude oil tank characterized by containing, in        mass, 0.001 to 0.2% C, 0.01 to 2.5% Si, 0.1 to 2% Mn, 0.03% or        less P, 0.007% or less S, 0.01 to 1.5% Cu, 0.001 to 0.3% Al,        0.001 to 0.01% N and one or both of 0.01 to 0.2% Mo and 0.01 to        0.5% W, with the balance consisting of Fe and unavoidable        impurities.    -   (2) A steel for a crude oil tank according to the item (1),        characterized by satisfying the following expression, in mass %;        Solute Mo+Solute W≧0.005%.

(3) A steel for a crude oil tank according to the item (1) or (2),characterized in that the carbon equivalent (Ceq.), in mass %, definedby the equation (1) is 0.4% or less;Ceq.=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+W+V)/5  (1).

(4) A steel for a crude oil tank according to any one of the items (1)to (3), characterized in that the Cr content is less than 0.1 mass %.

(5) A steel for a crude oil tank according to any one of the items (1)to (4), characterized by further containing, in mass, 0.1 to 3% Niand/or 0.1 to 3% Co.

(6) A steel for a crude oil tank according to any one of the items (1)to (5), characterized by further containing, in mass, one or more of0.01 to 0.3% Sb, 0.01 to 0.3% Sn, 0.01 to 0.3% Pb, 0.01 to 0.3% As and0.01 to 0.3% Bi.

(7) A steel for a crude oil tank according to any one of the items (1)to (6), characterized by further containing, in mass, one or more of0.002 to 0.2% Nb, 0.005 to 0.5% V, 0.002 to 0.2% Ti, 0.005 to 0.5% Ta,0.005 to 0.5% Zr and 0.0002 to 0.005% B.

(8) A steel for a crude oil tank according to any one of the items (1)to (7), characterized by further containing, in mass, one or more of0.0001 to 0.01% Mg, 0.0005 to 0.01% Ca, 0.0001 to 0.1% Y, 0.005 to 0.1%La and 0.005 to 0.1% Ce.

(9) A steel for a crude oil tank according to any one of the items (1)to (8), characterized in that the area percentage of microscopicsegregation portions where the Mn concentration is 1.2 times or more theaverage Mn concentration in the steel is 10% or less.

(10) A method for producing a steel for a crude oil tank according toany one of the items (1) to (9), characterized in that, in the event ofapplying accelerated cooling after hot rolling a slab containingcomponents according to any one of the items (1) to (8), the averagecooling rate of the accelerated cooling is in the range from 5 to 100°C./sec., the accelerated cooling end temperature is in the range from600° C. to 300° C., and the cooling rate in the temperature range fromthe accelerated cooling end temperature to 100° C. is in the range from0.1 to 4° C./sec.

(11) A method for producing a steel for a crude oil tank, characterizedby applying tempering or annealing at 500° C. or lower to a steelproduced by the method according to the item (10).

(12) A method for producing a steel for a crude oil tank according toany one of the items (1) to (9), characterized in that, in the event ofapplying normalizing after hot rolling a slab containing componentsaccording to any one of the items (1) to (8), the heating temperature ofthe normalizing is in the range from the Ac₃ transformation temperatureto 1,000° C. and the average cooling rate in the temperature range from700° C. to 300° C. is in the range from 0.5 to 4° C./sec.

(13) A method for producing a steel for a crude oil tank characterizedby applying tempering or annealing at 500° C. or lower to a steelnormalized according to the item (12).

(14) A method for producing a steel for a crude oil tank according toany one of the items (10) to (13), characterized by, before hot rollinga slab containing components according to any one of the items (1) to(8), applying diffusion heat treatment to the slab at a heatingtemperature of 1,200 to 1,350° C. and for a retention time of 2 to 100hr.

(15) A crude oil tank characterized in that the floor plate, deck plate,side walls and structural members thereof are made wholly or partiallyof a steel for a crude oil tank according to any one of the items (1) to(9).

(16) A method for protecting a crude oil tank against corrosioncharacterized by removing, either mechanically or chemically,hot-rolling scale on the surface of a crude oil tank according to theitem (15) and exposing the base steel substrate.

(17) A method for protecting a crude oil tank against corrosionaccording to the item (16), characterized by forming one or more layersof a coating film 10 μm or more in thickness on the surface afterhot-rolling scale is removed mechanically or chemically.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship between a local corrosionrate of progress and the Mo content of Fe—Cu—Mo steels.

FIG. 2 is a graph showing the relationship between a local corrosionrate of progress and the Cu content of Fe—Cu—Mo steels.

FIG. 3( a) is a graph showing the relationship between a local corrosionrate of progress and the P content of Fe—Cu—Mo steels.

FIG. 3( b) is a graph showing the relationship between a local corrosionrate of progress and the S content of Fe—Cu—Mo steels.

FIG. 4 is a graph showing the relationship between a local corrosionrate of progress and the Al content of Fe—Cu—Mo steels.

FIG. 5 is a schematic configuration diagram of a corrosion testapparatus.

FIG. 6 is a graph explaining the temperature cycle imposed on testpieces.

BEST MODE FOR CARRYING OUT THE INVENTION

The measures to be taken so as to overcome the aforementioned problemsand attaining the object of the present invention are hereafterexplained concretely.

Firstly, the component elements of a steel according to the presentinvention and their contents are explained. The contents of thecomponent elements are indicated herein in terms of mass %. C is to becontained by 0.001% or more because it is industrially very uneconomicalto decarbonize a steel to a carbon content of less than 0.001%. However,when C is used as a strengthening element, it is desirable to controlits content to 0.002% or more. On the other hand, when C is contained inexcess of 0.2%, weldability, the toughness of weld joints and otherproperties deteriorate to degrees unsuitable for a steel used for awelded structure. For this reason, the C content is limited in the rangefrom 0.001 to 0.2%. It is more desirable for the C content to be 0.18%or less from the viewpoint of welding operability. A C content in therange from 0.05 to 0.15% is yet more desirable, especially for mildsteels for marine vessels (of a yield stress of 240 N/mm² class),high-tensile steels (of a yield stress of 265, 315, 355, or 390 N/mm²class) and high-tensile steels for marine vessels. Since C is an elementthat somewhat lowers the local corrosion resistance of the floor plateof a crude oil tank, a C content desirable from the viewpoint ofcorrosion resistance is 0.15% or less.

Si is indispensable as a deoxidizing element, and its content must be0.01% or more so as to obtain a sufficient deoxidizing effect. Si is anelement effective in improving resistance to general corrosion and alsofor enhancing, though only slightly, resistance to local corrosion. Inorder to secure these effects, it is desirable to add Si by 0.1% ormore. On the other hand, when Si is added excessively, hot-rolling scalebecomes adhesive (scale exfoliates) and defects caused by hot-rollingscale increase. For this reason, the upper limit of Si content is set at2.5% in the present invention. In particular, when a steel is requiredto have higher weldability and toughness of base material and weld jointin addition to corrosion resistance, it is desirable to set the upperlimit at 0.5%.

0.1% or more Mn is required for securing steel strength. However, an Mncontent exceeding 2% is unacceptable, because weldability deterioratesand sensitivity to intergranular brittleness is increased. For thisreason, the Mn content is limited in the range from 0.1 to 2% in thepresent invention. It should be noted that since C and Mn are elementshaving little influence on corrosion resistance, it is possible toadjust the carbon equivalent by properly adjusting the content(s) of Cand/or Mn when the carbon equivalent has to be controlled within acertain range, especially for welded structure use.

P is an impurity element, and when its content is more than 0.03%, thelocal corrosion rate of progress increases and weldability deteriorates.For this reason, the P content is limited to 0.03% or less. When the Pcontent is 0.0153 or less, good effects are obtained, especially incorrosion resistance and weldability and for this reason, it isdesirable to control the P content to 0.015% or less. It is moredesirable to control the P content to 0.005% or less, because by sodoing, corrosion resistance is further improved, although productioncosts increase.

S is also an impurity element, and when its content is more than 0.007%,the local corrosion rate of progress increases, the amount of sludgeformed tends to increase, and mechanical properties, particularlyductility, deteriorate remarkably. For these reasons, the upper limit ofS content is set at 0.007%. The smaller the S content, the better thecorrosion resistance and mechanical properties. Therefore, it is moredesirable to control the S content to 0.005% or less.

Cu is effective in improving resistance to general corrosion as well asto local corrosion when it is added by 0.01% or more in combination withMo and W. Further, Cu is effective in decreasing the formation of solidsulfur when it is added by 0.03% or more. However, adverse effects suchas increase in slab surface cracking and deterioration of the toughnessof a weld joint become apparent when the Cu content is more than 1.5%.For this reason, the upper limit of a Cu content is set at 1.5% in thepresent invention. When Cu is added in excess of 0.5%, the enhancementof corrosion resistance becomes virtually saturated. Therefore, when itis intended to decrease the progress of local corrosion of the floorplate of a crude oil tank, a desirable Cu content is in the range from0.01 to 0.5%. When Cu is added by 0.2% or more, its effect of decreasingthe formation of sludge becomes virtually saturated. Therefore, when asteel is used for the upper deck of a crude oil tank, a preferable cucontent is in the range from 0.03 to less than 0.2% in consideration ofoperability.

Al is an element indispensable for suppressing the progress of localcorrosion when it is added together with Cu and Mo and/or W. Also, Alforms AlN and is an element effective in fractionizing austenite crystalgrains by AlN in the heating of a base material. Further, Al is a usefulelement, since it has the effect of suppressing the formation of acorrosion product containing solid sulfur. In order to secure theseeffects, an Al content of 0.001% or more is necessary. On the otherhand, when Al is contained in excess of 0.3%, coarse oxide forms,deteriorate ductility and toughness. For this reason, the Al content hasto be limited in the range from 0.001 to 0.3%. It is more desirable toadd Al by 0.02% or more so as to obtain sufficient effects of enhancingcorrosion resistance and decreasing the formation of a corrosion productcontaining solid sulfur. The corrosion resistance improvement effect ofAl is virtually saturated when it is added in excess of 0.1%, and thus amore desirable Al content range is from 0.02 to 0.10%.

N is undesirable because it adversely affects ductility and toughnesswhen it exists in a solid solution state. However, because N iseffective in fractionizing austenite grains and enhancing precipitationstrengthening when it combines with V, Al and Ti, it is effective inenhancing mechanical properties as long as its content is small. It isindustrially impossible to completely remove N from a steel andtherefore the reduction of N exceeding a necessary limit undesirablyimposes excessive burdens on production processes. For this reason, thelower limit of N content is set at 0.001% as a level that allows adverseeffects on ductility and toughness, industrial control and burdens onproduction processes. N has an effect of improving corrosion resistancesomewhat. However, when N is contained excessively, solute N increasesand ductility and toughness are likely to deteriorate. For this reason,the upper limit of N content is set at 0.01% as a tolerable level.

Mo and W are useful elements in local corrosion resistance, like Cu.When they are added in combination with 0.01% or more Cu, the effect ofdecreasing a local corrosion rate of progress is conspicuous. Mo and Wshow substantially the same effects. It is necessary to add Mo by 0.01to 0.2% and/or W by 0.01 to 0.5%. When Mo or W is added by 0.01% ormore, the effect of improving local corrosion resistance is conspicuous.On the other hand, when Mo is added by more than 0.2% or W by more than0.5%, local corrosion resistance deteriorates rather than improve, andweldability and toughness also deteriorate. For this reason, the Mocontent and W content are limited in the ranges from 0.01 to 0.2% andfrom 0.01 to 0.54, respectively. It should be noted that, in order tosuppress the formation of precipitates and steadily secure the amountsof Mo and W in solid solution, it is more desirable to set the upperlimits of the contents of Mo and W at lower than 0.1 and 0.05%,respectively. Further, a more desirable range of Mo addition is from0.01 to 0.08%, because a remarkable improvement in local corrosionresistance is realized with a smaller amount of addition. A yet betterrange of Mo addition is from 0.03 to 0.07% in consideration ofproduction stability. With respect to W, a more desirable range ofaddition amount is from 0.01 to less than 0.05%, because a remarkableimprovement in local corrosion resistance is realized with a smalleramount of addition.

While the aforementioned ranges of Mo and W contents are essentialrequirements, in order to achieve the effect of improving localcorrosion resistance more efficiently, it is necessary to secure morethan a certain amount of Mo and W in solid solution while their contentsare maintained within the above-required ranges. This is because, wheneither Mo or W forms coarse precipitates, portions depleted of theelement are formed around the precipitates and the effect of improvinglocal corrosion resistance is impaired. For this reason, it is necessarythat either Mo or W be distributed in a steel as uniformly as possible.Solute Mo and W have substantially identical effects on local corrosionresistance, and as long as the total amount of both the elements insolid solution is 0.005% or more, local corrosion resistance is greatlyimproved. It is not necessary to specify an upper limit of the totalamount of the solute Mo and W for obtaining the effects of the presentinvention. On the other hand, a steel is strengthened by solid solution,and in order to obtain an adequate strength economically, it isdesirable to set the upper limit of the total amount of both theelements in solid solution at 0.5% or less.

Here, the total amount of Mo and W in solid solution cited in thepresent invention as effective for improving local corrosion resistanceis defined by a value obtained by subtracting the amount of precipitatesobtained through extraction residue analysis from the total content ofthe elements. This is because very fine precipitates that are regardedas being solute by extraction residue analysis can be viewed as beinguniformly distributed in a steel like solute elements, and they workpositively to improve corrosion resistance.

The fundamental requirements regarding the chemical composition of asteel according to the present invention and the reasons for definingthem are described above. The present invention further specifies theconditions of elements that may be added to a steel optionally with theaim of improving various steel properties.

First, when it is necessary to give special consideration to weldabilityand the toughness of a weld joint, a carbon equivalent (Ceq.) defined bythe equation (1) is controlled to 0.4% or less;Ceq.=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+W+V)/5  (1).

The equation (1) is a carbon equivalent formula including W, which is animportant element in the present invention. When a carbon equivalentaccording to the equation (1) is 0.4% or less, the hardening of a weldheat-affected zone (HAZ) is inhibited and resistance to low-temperaturecracking and the toughness of HAZ are surely improved. For this reason,it is desirable to control the carbon equivalent to 0.4% or less. Whenthe carbon equivalent is too large in excess of 0.4%, resistance tolow-temperature cracking and the toughness of a HAZ, or even the stresscorrosion cracking of a HAZ, may deteriorate in some combination ofcomponents. It is not necessary to specify a lower limit of carbonequivalent for obtaining the effects of the present invention. However,it is preferable to set the lower limit at 0.36% in order to obtainexcellent toughness in the low temperature range from 0 to −40° C.

Cr is a strengthening element and it may be added for adjusting steelstrength as required. However, Cr is the element that most increases thelocal corrosion rate of progress and thus should be as low as possible.When the Cr content is 0.1% or more, local corrosion resistance in acrude oil environment deteriorates and the formation of solid sulfur isaccelerated to some extent. Therefore, a Cr content of 0.1% or more isnot desirable in the present invention. As a conclusion, it is desirablenot to add Cr intentionally or, if it is added either unavoidably orintentionally, to control the Cr content to less than 0.1%.

Ni and Co are elements effective in enhancing the toughness of a basematerial and a HAZ. They are effective also in improving corrosionresistance and suppressing sludge formation in a steel containing Cu andMo. Either Ni or Co begins to exhibit tangible effects of improvingtoughness and corrosion resistance only when added by 0.1% or more. Onthe other hand, an excessive addition of either of them exceeding 3% isuneconomical because of its high price and causes weldability todeteriorate. For this reason, when Ni and/or Co are/is added, thecontent of each of them is limited in the range from 0.1 to 3% in thepresent invention.

When any one of Sb, Sn, As, Bi and Pb is added by 0.01% or more, theprogress of local corrosion is further suppressed. For this reason, theymay be added as required. In this case, the lower limit of content ofeach of them is set at 0.01%. However, when any of them is added inexcess of 0.3%, the above effect is saturated and other steel propertiesmay deteriorate. For this reason and in consideration also of economicalefficiency, the upper limit of a content of each of the above elementsis set at 0.3%. A more desirable content range is from 0.01 to 0.15%each.

Nb, V, Ti, Ta, Zr and B are elements effective in strengthening steelwith a small addition amount and, as such, any of them may be added asrequired, principally for adjusting steel strength. In order for eachelement to obtain a tangible effect, the contents of each should be:0.002% or more for Nb; 0.005% or more for V; 0.002% or more for Ti;0.005% or more for Ta; 0.005% or more for Zr; or 0.0002% or more for B.On the other hand, when more than 0.2% Nb, more than 0.5% V, more than0.2% Ti, more than 0.5% Ta, more than 0.5% Zr or more than 0.005% B isadded, adversely, toughness is markedly lowered. For this reason, whenany of these elements is added as required, each of the contents islimited in the ranges: from 0.002 to 0.2% for Nb; from 0-005 to 0.5% forV; from 0.002 to 0.2% for Ti; from 0.005 to 0.5% for Ta; from 0.005 to0.5% for Zr; or from 0.0002 to 0.005% for B.

Mg, Ca, Y, La and Ce are effective in controlling the shape ofinclusions and enhancing ductility and the HAZ toughness of alarge-heat-input weld joint. They have also an effect of stabilizing Sand thus suppressing the formation of sludge, though this is onlyslight. For this reason, they are added as required. The lower limits ofthe contents of those elements are defined in the present invention onthe basis of the smallest contents with which a tangible effect isobtained, and the lower limits are as follows: 0.0001% for Mg; 0.0005%for Ca; 0.0001% for Y; 0.005% for La; and 0.005% for Ce. The upperlimits, on the other hand, are defined on the basis of whether coarseinclusions form and degrade mechanical properties, especially ductilityand toughness, and from this viewpoint, the upper limits according tothe present invention are as follows: 0.01% for Mg and Ca; and 0.1% forY, La and Ce. When Mg or Ca is added by 0.0005% or more, it brings aboutan additional effect of suppressing the acidification of the inside of alocal corrosion pit and, for this reason, a preferable range of each ofthe two elements is from 0.0005 to 0.01%.

The reasons for specifying the chemical composition according to thepresent invention have been explained above. Additionally, the presentinvention specifies the microscopic segregation conditions of a steelunder some conditions of a slab, as required. This is because, in orderto obtain good local corrosion resistance, it is necessary that theelements that bring about local corrosion resistance be distributed asuniformly as possible in the entire steel. To this end, it is desirablethat the degree of microscopic segregation be low. In addition, when theconcentration of a component element, even other than those contributingto the improvement of local corrosion resistance, fluctuates, then localcorrosion is accelerated because of the fluctuation. For this reason,the microscopic segregation conditions of a steel are specified in thepresent invention as required. Since the condition of microscopicsegregation is represented virtually by the segregation of Mn, when themicroscopic segregation condition is to be specified, in the presentinvention, the area percentage of microscopic segregation portions wherethe Mn concentration is 1.2 times or more the average Mn concentrationin the steel is set at 10% or less.

The reason the microscopic segregation condition is specified as aboveis that when the concentration of a component element at a portion isconspicuously high in excess of 1.2 times the average concentration, theconcentration difference from the portions depleted of the elementbecomes significant from the viewpoint of corrosion resistance. It hasbeen confirmed on the basis of precise experiments that corrosionresistance is not adversely affected substantially as long as the ratioof the concentrated portions is 10% or less in terms of area percentagein a section surface. Thus, in the present invention, the microscopicsegregation condition is evaluated in terms of the concentration of Mn,and the area percentage of microscopic segregation portions where the Mnconcentration is 1.2 times or more the average Mn concentration in thesteel is set at 10% or less. A smaller area percentage of microscopicsegregation portions is preferable and the optimum lower limit thereofis 0%.

Microscopic segregation is measured by using an X-ray microanalyzer andthe area percentage of the portions where the Mn concentration is 1.2times or more the average Mn concentration is calculated from aconcentration map. The measurement is done on a section perpendicular tothe plate surface at several points along the thickness of a steel platefrom immediately below a plate surface to the thickness center, and therequirement of the present invention should be satisfied at all themeasurement points.

Next, explanations are given on the requirements of the presentinvention regarding the steel production methods for satisfying theabove-explained requirements of a steel according to the presentinvention, mainly for securing the solid solution amount of Mo and W andcontrolling the state of microscopic segregation. It should be noted,however, that the requirements of a steel according to the presentinvention may be attained by any means, that is to say, the means tosatisfy the requirements is not limited to the production methodsstipulated in the present invention.

In the present invention, production methods mainly for securing theamount of Mo and W in solid solution are roughly classified in thefollowing two methods: {circle around (1)} a method employing athermo-mechanical treatment, or {circle around (2)} a method employing anormalizing treatment after hot rolling. Further, a production methodfor controlling microscopic segregation requires {circle around (3)} amethod employing a diffusion heat treatment prior to hot rolling inaddition to the above both methods of {circle around (1)} and {circlearound (2)}. The requirements of the above methods are summarizedhereafter.

{circle around (1)} In the event of applying a thermo-mechanicaltreatment wherein an accelerated cooling is applied after hot rolling:the average cooling rate of the accelerated cooling is in the range from5 to 100° C./sec.; the accelerated cooling end temperature is in therange from 600° C. to 300° C.; the cooling rate in the temperature rangefrom the accelerated cooling end temperature to 100° C. is in the rangefrom 0.1 to 4° C./sec.; and, as required, a tempering or annealingtreatment may be applied at 500° C. or lower after the completion of thehot rolling and accelerated cooling.

{circle around (2)} In the event of applying a normalizing treatmentafter hot rolling: the heating temperature of the normalizing treatmentis in the range from the Ac₃ transformation temperature to 1,000° C.;the average cooling rate from 700° C. to 300° C. is 0.5 to 4° C./sec.;and, as required, a tempering or annealing treatment may be applied at500° C. or lower.

{circle around (3)} A diffusion heat treatment is applied at a heatingtemperature of 1,200° C. to 1,350° C. and for a retention time of 2 to100 hr. prior to hot rolling.

First, the method of the item {circle around (1)} will be explained.

In the event of applying a thermo-mechanical treatment whereinaccelerated cooling is applied after hot rolling, the conditions ofcooling including the accelerated cooling after hot rolling should bespecified for securing a required amount of Mo and W in solid solution.

It is necessary that the average cooling rate of the acceleratedcooling, which is done by water cooling or other, be in the range from 5to 100° C./sec., the accelerated cooling end temperature be in the rangefrom 600° C. to 300° C. and the cooling rate in the temperature rangefrom the accelerated cooling end temperature to 100° C. be in the rangefrom 0.1 to 4° C./sec.

The reasons the lower limit of the cooling rate of the acceleratedcooling is set at 5° C./sec are that, if the cooling rate is lower than5° C./sec., the improvement in strength and toughness is not conspicuousand the application of the accelerated cooling is not recommended andthere is the possibility of Mo and W forming precipitates during thecooling, making it difficult to secure the solid solution amount of Moand W. On the other hand, a larger cooling rate of the acceleratedcooling is preferable in terms of the improvement in strength and thesuppression of the precipitation of Mo and W. However, when the coolingrate is more than 100° C./sec., a flatness of a steel plate is likely todeteriorate. For this reason, the upper limit of cooling rate of theaccelerated cooling is set at 100° C./sec.

The accelerated cooling is finished in the temperature range from 600°C. to 300° C. If an accelerated cooling end temperature is higher than600° C., then, even if a cooling rate after the end of the acceleratedcooling is controlled in the range specified in the present invention,Mo and W form precipitates after the accelerated cooling and asufficient solid solution amount of Mo and W cannot be secured. Such acase is not desirable, because there is a risk that the corrosionresistance will be somewhat inferior to the case where the solidsolution amount of Mo and W specified in the present invention issecured. On the other hand, if the accelerated cooling end temperatureis lower than 300° C., undesirably, a toughness level requiredespecially of a steel for a welded structure is secured in some chemicalcompositions, residual stress increases, and a flatness of a steel plateis likely to deteriorate.

It should be noted that since the influence of an accelerated coolingcommencement temperature on the solid solution amount of Mo and W isvery small in comparison with an accelerated cooling end temperature, itis not necessary to specify an accelerated cooling commencementtemperature. However, it is desirable to commence the acceleratedcooling immediately after completing hot rolling in order not to allowstrength and toughness to decrease. No significant problem arises if theaccelerated cooling is commenced, as a guideline, at the Ar₃transformation temperature or higher.

In order to secure the amount of Mo and W in solid solution steadily, itis necessary to give due consideration to the cooling after finishingthe accelerated cooling. If cooling in the temperature range from anaccelerated cooling end temperature to 100° C. is slow at a cooling ratelower than 0.1° C./sec., Mo and W may possibly form carbonitridesthereof during such slow cooling. For this reason, in the cases where,for example, the thickness of a steel plate is large and the coolingrate by air cooling inevitably becomes lower than 0.1° C./sec., it isnecessary to control the cooling rate so as to be 0.1° C./sec. or higherby means such as shower cooling or gas cooling. A higher rate of coolingis more reliable in the effect of securing the amount of Mo and W insolid solution. However, if the cooling rate is higher than 4° C./sec.,the effect is saturated and, also, the cooling rate is differentiatedfrom a cooling rate in the range from 5 to 100° C./sec. controlled inthe accelerated cooling after hot rolling, there is a risk thatdeterioration of toughness, increase in residual stress and otheradverse effects will become obvious. For this reason, in the presentinvention, the upper limit of cooling rate is set at 4° C./sec.

The above-explained hot rolling and cooling process may be the finalproduction process of a steel according to the present invention, but atempering or annealing treatment may be applied thereafter for thepurpose of adjusting material properties. In order to suppress theprecipitation of Mo and W during the tempering or annealing treatmentand secure the amount of Mo and W in solid solution, it is necessary tolimit the temperature in the treatment to 500° C. or lower.

Next, the method of the item {circle around (2)} is explained.

The method of the item {circle around (2)} is the method according tothe present invention in the case where a steel is produced throughnormalizing. Like in the method of the item {circle around (1)}, theconditions of normalizing should be specified for suppressing theprecipitation of Mo and W during a normalizing process and securing arequired amount of Mo and W in solid solution. It should be noted that,at the time when a steel transforms into single-phase austenite in theheating stage of normalizing, the influences of the thermal history ofthe steel are dissipated theretofore and for this reason, the conditionsof hot rolling prior to the normalizing not have to be specified.Therefore, the hot rolling may be normal continuous hot rolling, acontrolled rolling, or a thermo-mechanical processing accompanyingaccelerated cooling. The history before and after the hot rolling nothave to be particularly specified, either.

The basic requirements of the production method of the item {circlearound (2)} are that, in the event of applying a normalizing treatmentafter hot rolling, the heating temperature of the normalizing treatmentis in the range from the Ac₃ transformation temperature to 1,000° C. andthe average cooling rate at the cooling stage from 700 to 300° C. is 0.5to 4° C./sec.

If a heating temperature is lower than the Ac₃ transformationtemperature, it is impossible to sufficiently dissolve the parts of Moand W that have precipitated before the normalizing treatment and, as aresult, corrosion resistance deteriorates. Another adverse effect isthat the metallographic structure becomes uneven, and the strength andductility deteriorate. On the other hand, if the heating temperature ishigher than 1,000° C., austenite grains become coarse by the heating,the final transformation structure becomes coarse as a consequence, andtoughness is lowered significantly. For this reason, the heatingtemperature of the normalizing treatment is specified to be in the rangefrom the Ac₃ transformation temperature to 1,000° C. in the presentinvention.

In an ordinary normalizing process, the cooling after heating andretention is done by air cooling. However, in the present invention, inthe case where air cooling is too slow to secure the amount of Mo and Win solid solution, it is necessary to control the cooling rate so thatthe average cooling rate in the range from 700° C. to 300° C. may be 0.5to 4° C./sec. by any practical means. If the average cooling rate in therange from 700° C. to 300° C. is lower than 0.5° C./sec., Mo and W formprecipitates during the cooling and the possibility that the solidsolution amount of Mo and W in the range specified in the presentinvention is not secured becomes significantly high. A higher coolingrate of normalizing is more reliable in terms of securing the solidsolution amount of Mo and W. However, if the cooling rate exceeds 4°C./sec., the effect is saturated and there is a risk that deteriorationof toughness, increase in residual stress and other adverse effects willbecome obvious. For this reason, in the present invention, the upperlimit of cooling rate is set at 4° C./sec. A normalizing treatmentwithout an accelerated cooling is different from the method of the item{circle around (1)} and, for this reason, a cooling rate in thetemperature range of lower than 300° C. is not specified in the presentinvention. However, such slow cooling that an average cooling rate inthe temperature range from 300° C. to 100° C. is far lower than 0.1°C./sec. is undesirable.

The above-explained normalizing process may be the final productionprocess of a steel according to the present invention, but a temperingor annealing treatment may be applied thereafter for the purpose ofadjusting material properties. In order to suppress the precipitation ofMo and W during the tempering or annealing treatment and secure theamount of Mo and W in solid solution, it is necessary to limit thetemperature in the treatment to 500° C. or lower.

Finally, the production method of the item {circle around (3)} will beexplained. The method of the item {circle around (3)} is a means tosatisfy the requirements of the present invention regarding microscopicsegregation, and the basic requirements thereof are that, prior to hotrolling, a diffusion heat treatment is applied at a heating temperatureof 1,200° C. to 1,350° C. and for a retention time of 2 to 100 hr. inthe heating temperature range. Elements that have segregatedmicroscopically are diffused by a diffusion heat treatment and thus theincrassation of the microscopic segregation portions is lowered. If theheating temperature of the diffusion heat treatment is lower than 1,200°C., the diffusion rates of the elements are too low to obtain asufficient diffusion effect with a practical retention time. As theheating temperature increases, though the diffusion rate also increasesfavorably for the mitigation of segregation, austenite grains grow toocoarse by the heating, and there is a risk that a coarse structure willremain through the hot rolling and heat treatment after the diffusiontreatment and adversely affect the mechanical properties of the steel,and the possibility of a rough surface forming on the steel sheetsurface increases. For this reason, in the present invention, the upperlimit of heating temperature of the diffusion heat treatment is set at1,350° C. in consideration of practically acceptable degrees of theabove adverse effects.

When the heating temperature of the diffusion heat treatment ismaintained in the range from 1,200° C. to 1,350° C., a retention time of2 hr. or more is required for sufficiently dissipating microscopicsegregation. The longer the retention time, the more the diffusionprogresses. However, as far as the microscopic segregation usually seenin a steel ingot or slab is concerned, a sufficient effect of adiffusion heat treatment is obtained after a retention time of 100 hr.For this reason, in consideration also of economical efficiency, theupper limit of retention time of the diffusion treatment is set at 100hr. in the present invention.

It is not necessary to specify the conditions of cooling after theretention for 2 to 100 hr. at 1,200° C. to 1,350° C. However, ifdiffusion is expected to continue during the cooling, it is desirable toadopt slow cooling at a cooling rate equal to or less than that of aircooling.

Here, if it is intended to apply a diffusion heat treatment after hotrolling, the capacity of a heat treatment furnace may possibly be apractical problem, since the dimensions of a steel becomes larger afterhot rolling and it is necessary to fractionize a metallographicstructure that has once coarsened by the diffusion heat treatment.Therefore, the present invention stipulates that a diffusion heattreatment be applied before hot rolling. However, in the method of theitem {circle around (2)}, if the above problems do not arise, then adiffusion heat treatment may be applied after hot rolling and before thenormalizing treatment. In this case, the effects of the diffusiontreatment are not in the least reduced.

Next, a crude oil tank made of a steel according to the presentinvention will be described. When a steel according to the presentinvention is used wholly or partially for the floor plate, deck plate,side walls and structural members of a crude oil tank, the rate ofprogress of local corrosion occurring inside the tank is significantlyreduced, and as a consequence the frequency of repair work of the tankis reduced and safety is enhanced. The effects obtained with a crude oiltank for which a steel according to the present invention is used isexplained below in further detail in comparison with another for whichan ordinary steel is used.

High-concentration brine contained in crude oil separates and settles atthe bottom of an oil tank, and local corrosion occurs at variousportions of the tank. Local corrosion inevitably occurs, especially atthe floor plate and side walls. When a steel according to the presentinvention is used for those portions of a tank where local corrosionoccurs or for all of it in accordance with the structure of the oiltank, the local corrosion rate of progress is significantly reduced. Acrude oil tank excellent in durability and economical efficiency can beconstructed by using a steel according to the present inventionselectively for those portions that cannot be thoroughly washed forstructural reasons and are continuously exposed to high-concentrationbrine.

As a general rule, a crude oil tank is legally obliged to undergoperiodical overhaul inspections wherein the positions and depths oflocal corrosion are inspected and pitting corrosion portions deeper thana prescribed figure are repaired by a method such as padding welding. Inthe case of a crude oil tank using a steel according to the presentinvention, as long as the interval of the periodical inspections is keptunchanged, the number of pitting corrosion that requires repair isdrastically decreased, and the costs and time required for repair workare significantly reduced. Further, even if progressive local corrosionat some portion of such a tank is overlooked in inspection andeventually not repaired, the probability of the local corrosiondeveloping into a through hole leading to an oil leakage accident isless in comparison with a crude oil tank for which an ordinary steel isused, when the steel thickness is identical. Thus, the present inventioncontributes to the enhancement of the safety of a crude oil tank. Theuse of a steel according to the present invention makes it possible toconstruct a crude oil tank excellent in economical efficiency and safetywith the same level of welding workability and mechanical properties ofa steel as in the case where an ordinary steel is used. In addition,when a steel according to the present invention is used for the deck orceiling plate of a crude oil tank, the formation of sludge at thereverse side of a deck or ceiling plate is significantly reduced, andconsequently, the costs for recovering the sludge can be reduced aswell.

The effects of the present invention are explained below in furtherdetail on the basis of examples. It should be noted that the presentinvention should not be interpreted as being limited to the examplesdescribed below.

Example

Specimen steels were melted and refined with a vacuum melting furnace ora converter, cast into ingots or slabs and hot rolled into steel plates.Table 1 shows the chemical compositions of the specimen steels and Table2 the production conditions of the steel plates. In producing the steelplates, the conditions of diffusion heat treatment, hot rolling,normalizing and tempering and the combination of these processes werechanged so that the effects of the production method according to thepresent invention might be clearly shown. Note that Table 2 also showsthe measurement results of the amounts of Mo and W in solid solution andthe conditions of microscopic segregation of Mn in the specimen steelplates. The amounts of solute Mo and W were measured by extractionresidue analysis using through-thickness test pieces of the specimenplates removed of oxide skin. The microscopic segregation was measuredwith an X-ray microanalyzer on a section perpendicular to the surface ofthe steel plate at three points, namely 1 mm from the surface, ¼ of theplate thickness and at the thickness center, and the area percentage ofthe portions where the Mn concentration was 1.2 times or more theaverage Mn concentration was calculated from a concentration map byimage analysis.

Table 3 shows the mechanical properties (strengths and 2-mm V-notchCharpy impact test results) of the specimen steel plates and the maximumhardness of HAZ, as an indicator of their weldability. Tables 4 and 5show the results of corrosion tests: Table 4 shows the results of teststo evaluate mainly local corrosion resistance, and Table 5 the resultsof tests to evaluate mainly general corrosion resistance and sludgeformation behaviors.

With respect to the mechanical properties of the specimen steel plates,strength and toughness were measured through round bar tensile tests and2-mm V-notch Charpy impact tests, and the test pieces were cut out fromthe thickness center so that their longitudinal direction was at rightangles to the rolling direction of the specimen plates. The tensiletests were carried out at the room temperature. The 2-mm V-notch Charpyimpact tests were carried out at different temperatures, and thefracture appearance transition temperature calculated from thetransition curve was used as an indicator of toughness.

The maximum hardness of HAZ was tested according to JIS Z 3101 withoutpreheating.

The conditions of the tests, which are shown in Table 4, to evaluatemainly local corrosion resistance are as follows:

Test pieces 40 mm in length, 40 mm in width and 4 mm in thickness werecut out so that the thickness center of the test pieces coincided withthe ¼ thickness of the specimen steel plates. All the surfaces of thetest pieces were mechanically polished, then wet polished to #600 finishby the surface roughness code, and then their edge faces were coatedwith paint, leaving the top and bottom 40 mm×40 mm faces withoutcoating. Then, the test pieces were immersed in two different corrosiveliquids, namely 10- and 20-mass-% aqueous solutions of NaCl, whosevalues of pH had been adjusted to 0.2 with hydrochloric acid. Otherimmersion conditions were the liquid temperature of 30° C. and theimmersion time of 24 hr. to 4 weeks, and then corrosion weight loss wasmeasured for evaluating the corrosion rate. The compositions of thecorrosive liquids were those simulating the conditions of theenvironments where local corrosion occurred to real steel structures andtherefore, as the corrosion rate of a steel at the corrosion testdecreases, the rate of progress of local corrosion of the steel in areal environment decreases.

The conditions of the tests, which are shown in Table 5, to examinegeneral corrosion resistance and the formation behavior of sludge are asfollows:

Test pieces 40 mm in length, 40 mm in width and 4 mm in thickness werecut out so that the thickness center of the test pieces coincided withthe ¼ thickness of the specimen steel plates. All the surfaces of thetest pieces were mechanically polished, then wet polished to #600 finishby the surface roughness code, and their edge faces and one of the topand bottom 40 mm×40 mm faces were coated with paint, leaving the other40 mm×40 mm face without paint coating. The corrosion rates and theformation rates of sludge composed mainly of solid sulfur of thespecimen steels were evaluated with a test apparatus as schematicallyshown in FIG. 5. Table 6 shows the composition of the atmosphere gasthat was used for the above corrosion tests.

The dew point of the atmosphere gas was adjusted to a prescribedtemperature (30° C.) by making the gas pass through a dew pointadjustment water tank 2 and then the gas was introduced to a testchamber 3. The surface of each of the test pieces 4 left without paintcoating was coated with an aqueous solution of NaCl prior to the testsso that the deposition amount of NaCl was 1,000 mg/m² and then, afterdrying, the test pieces were placed horizontally on aconstant-temperature heating plate 5 in the test chamber. Thetemperature cycle shown in FIG. 7, 20° C.×1 hr.+40° C.×1 hr., in total 2hr. per cycle, was repeated by controlling a heater controller 6 so thatwet and dry were repeated alternately at the surfaces of the testpieces. After 720 cycles, the rate of corrosion was evaluated fromcorrosion weight loss, and the rate of sludge formation from the mass ofcorrosion products that formed on the surface of each test piece. Here,it has been confirmed through chemical and x-ray analyses at preliminarytests that the corrosion products consist of iron oxyhydroxide (ironrust) and solid sulfur.

First, with respect to mechanical properties, it is clear from theresults shown in Table 3 that every one of the steel plate nos. A1 toA26, which satisfy the requirements of the present invention, hassufficiently good properties as a steel for a welded structure. Further,with respect to weldability, it is clear that every one of the steelplates of the invention samples that have a value of the carbonequivalent defined by expression (1) equal to or less than 0.4% exhibitsa maximum HAZ hardness of 300 or less in terms of Vickers hardness, andthus has good weldability.

It should be noted that, although the steel plate no. A25 is aninvention sample, the amount of solute Mo is smaller than two otherinvention samples (the steel plate nos. A1 and A11) of the same chemicalcomposition and therefore it is somewhat inferior in local corrosionresistance. Nevertheless, it is significantly superior in corrosionresistance to comparative samples.

Although the steel plate no. A26 satisfies the chemical compositionstipulated in the present invention, the total amount of Mo and W insolid solution is slightly smaller than two other invention samples (thesteel plate nos. A6 and A13) of the same chemical composition andtherefore it is somewhat inferior in local corrosion resistance.Nevertheless, it is significantly superior in corrosion resistance tocomparative samples.

From the local corrosion resistance shown in Table 4 and the generalcorrosion resistance and the amount of sludge formation shown in Table5, it has been clarified that: the corrosion rates and sludge formationrates of all the invention samples are suppressed to roughly ¼ times orless those of the comparative steel plate no. B1, which is of virtuallythe same chemical composition as an ordinary steel and does not containany of Cu, Mo and W, the indispensable elements of the presentinvention; and thus all the invention samples have remarkably improvedcorrosion resistance. With respect to local corrosion resistance shownin FIG. 4 in particular, further enhancement of local corrosionresistance is realized in those invention samples wherein microscopicsegregation is very little or it is reduced through diffusion heattreatment so that the area percentage of the microscopic segregationportions where the Mn concentration is 1.2 times or more the average Mnconcentration of the steel is 10% or less.

On the other hand, the steel plates nos. B1 to B9 are comparativesamples which are inferior in corrosion resistance to invention samples,because some of the requirements of the present invention are notsatisfied.

The steel plate no. B1 (slab no. 31) does not contain any of Cu, Mo andW, which are indispensable for decreasing local corrosion and theformation of sludge and, as a natural result, does not contain therequired amount of Mo and W in solid solution and, consequently, issignificantly inferior to the invention samples in any of localcorrosion resistance, general corrosion resistance and resistance tosludge formation.

The steel plate no. B2 (slab no. 32) contains Cu but neither Mo nor Wand, as a result, is significantly inferior to the invention samples inany of local corrosion resistance, general corrosion resistance andresistance to sludge formation.

The steel plate no. B3 (slab no. 33) contains Mo but not Cu, and failsto realize the effects of the present invention and, as a result, issignificantly inferior to the invention samples in any of localcorrosion resistance, general corrosion resistance and resistance tosludge formation.

The steel plate no. B4 (slab no. 34) contains an excessive amount of Crand, as a result, is inferior to the invention samples in corrosionresistance. The local corrosion resistance of this specimen, especiallyin a corrosive environment of a high salt concentration (correspondingto corrosion condition {circle around (2)} in Table 4), is significantlyinferior to that of an ordinary steel.

The steel plate no. B5 (slab no. 35) contains an excessive amount of Pand, as a result, is inferior to the invention samples in any of localcorrosion resistance, general corrosion resistance and resistance tosludge formation. This specimen shows a tendency toward a larger sludgeformation.

The steel plate no. B6 (slab no. 36) contains an excessive amount of Sand, as a result, is inferior to the invention samples in any of localcorrosion resistance, general corrosion resistance and resistance tosludge formation. This specimen also shows a tendency toward largersludge formation.

The steel plate no. B7 (slab no. 37) contains Al by an amount less thanthe lower limit stipulated in the present invention and; as a result, isinferior to the invention samples in local corrosion resistance. Thisspecimen also shows a tendency toward larger sludge formation.

The steel plate no. B8 (slab no. 38) contains an excessive amount of Aland, as a result, is inferior to the invention samples in localcorrosion resistance. This specimen also shows a tendency toward largersludge formation. The toughness is also poor.

The steel plate no. B9 (slab no. 39) contains an excessive amount of Moand, as a result, is inferior to the invention samples in localcorrosion resistance. This specimen also shows a tendency toward largersludge formation. The toughness and weldability are also poor.

From the examples described above, it is obvious that the presentinvention makes it possible to secure excellent general and localcorrosion resistance to such crude oil corrosion as caused in a steeloil tank for transporting or storing crude oil, and to suppress theformation of corrosion products (sludge) containing solid sulfur.

TABLE 1 Classifi- Slab Chemical components (mass %) cation no. C Si Mn PS Al N Cu Ni Co Cr Mo W Invention 1 0.15 0.33 1.13 0.010 0.008 0.0350.0035 0.26 — — 0.003 0.046 — sample 2 0.14 0.21 1.46 0.008 0.003 0.0460.0032 0.35 — — 0.012 0.078 — 3 0.09 0.19 1.37 0.008 0.002 0.016 0.00410.33 0.25 — 0.005 0.051 — 4 0.06 0.09 1.01 0.006 0.002 0.011 0.0036 0.350.65 — 0.003 0.075 — 5 0.11 0.25 1.48 0.006 0.004 0.019 0.0040 0.45 0.11— 0.005 — 0.044 6 0.11 0.29 1.33 0.009 0.003 0.037 0.0029 0.34 0.16 —0.003 0.030 0.031 7 0.10 0.26 1.35 0.011 0.004 0.020 0.0037 0.25 0.130.10 0.009 0.065 0.047 8 0.09 0.21 0.93 0.007 0.002 0.055 0.0031 0.270.96 — 0.006 0.200 — 9 0.05 0.18 1.32 0.008 0.003 0.010 0.0022 0.31 0.13— 0.003 0.052 — 10 0.07 0.23 1.05 0.010 0.001 0.023 0.0033 0.24 — 0.150.002 — 0.049 11 0.12 0.20 0.95 0.005 0.004 0.030 0.0040 0.49 0.29 —0.002 0.050 — 12 0.11 0.21 1.00 0.015 0.004 0.029 0.0039 0.15 0.09 —0.050 0.074 — 13 0.10 0.23 1.34 0.009 0.003 0.033 0.0045 0.09 — — 0.0200.020 — 14 0.11 0.22 1.15 0.005 0.003 0.025 0.0038 0.05 0.11 — 0.0100.055 — 15 0.17 0.20 1.00 0.005 0.005 0.027 0.0041 0.31 0.31 — 0.0100.069 — 16 0.12 0.20 0.95 0.005 0.004 0.030 0.0040 0.31 0.32 — 0.0100.030 — 17 0.12 0.20 0.95 0.005 0.004 0.030 0.0040 0.31 0.32 — 0.0100.051 — Comparative 31 0.15 0.54 1.16 0.015 0.005 0.037 0.0046 — — —0.005 — — sample 32 0.13 0.26 1.45 0.013 0.003 0.029 0.0055 0.51 0.16 —0.003 — — 33 0.12 0.34 1.47 0.010 0.002 0.030 0.0040 — — — 0.005 0.033 —34 0.13 0.51 0.95 0.015 0.003 0.026 0.0039 0.32 0.33 — 0.260 0.053 — 350.12 0.25 1.05 0.054 0.005 0.027 0.0041 0.39 0.38 — 0.010 0.075 0.050 360.13 0.28 1.25 0.020 0.025 0.030 0.0044 0.25 0.22 — 0.005 0.029 — 370.11 0.25 1.10 0.014 0.005 — 0.0045 0.21 0.21 — 0.005 0.105 0.125 380.11 0.25 1.10 0.015 0.007 0.450 0.0045 0.31 0.31 — 0.005 0.130 — 390.12 0.22 1.37 0.019 0.005 0.028 0.0041 0.49 0.45 — 0.090 0.310 — Equa-tion Classifi- Slab Chemical components (mass %) (1) cation no. Nb Ta VTi Zr B Sb Sn As Bi Mg Ca Y La Ce Ceq. Invention 1 — — — — — — — — — — —— — — — 0.365 sample 2 0.009 — — 0.012 — — — — — — — — — — — 0.425 30.015 — — 0.009 — — 0.03 — — — — 0.0018 — — — 0.368 4 0.010 — 0.0250.008 — 0.0013 — — — — — 0.0016 — — — 0.316 5 0.023 — 0.055 0.015 — — —0.05 0.04 — 0.0009 — — — — 0.415 6 — 0.08 — 0.008 — 0.0006 — — — — — —0.0011 — — 0.378 7 0.008 — 0.020 0.011 0.007 0.0003 — — — 0.05 — 0.0025— 0.005 — 0.379 8 — — 0.047 — — 0.0015 0.02 0.02 0.01 — — 0.0018 0.0021— — 0.378 9 0.006 — — 0.010 — — — 0.03 — 0.01 — 0.0009 0.0110 — 0.0080.310 10 0.006 0.06 — 0.009 0.009 — — 0.02 0.05 0.02 0.0015 0.01000.0080 0.009 — 0.271 11 0.006 — — 0.014 — — — — — — — — — — — 0.341 120.007 — — 0.013 — — — — — — — — — — — 0.317 13 0.010 — 0.020 — — — — — —— — — — — — 0.341 14 0.006 — — 0.012 — — — — — — — — — — — 0.325 150.006 — — 0.012 — — — — — — — — — — — 0.394 16 0.005 — — 0.012 — — — — —— — — — — — 0.328 17 0.006 — — 0.012 — — 0.09 — — — — — — — — 0.333Comparative 31 0.010 — — 0.011 — — — — — — — — — — — 0.344 sample 320.014 — — 0.013 — — — 0.03 — — — 0.0018 — — — 0.417 33 0.010 — — 0.005 —— — — — — — — — — — 0.373 34 0.015 — — 0.012 — — — — — — — — — — — 0.39435 — — — — — — — — — — — — — — — 0.373 36 — — — 0.010 — — — — — — — — —— — 0.376 37 — — — — — — — — — — — — — — — 0.368 38 — — — — — — — — — —— — — — — 0.362 39 — — — — — — — — — — — — — — — 0.491

TABLE 2 Slab production Diffusion heat treatment Hot rolling method(Note 1) conditions conditions Steel Slab Heating Retention CoolingReheating plate Slab thickness temperature time condition temperatureClassification no. no. Process route (nm) (° C.) (h) (Note 2) (° C.)Invention A1 1 Converter - continuous casting 200 — — — 1200 sample A2 2Converter - continuous casting 250 — — — 1250 A3 3 Converter -continuous casting 200 1300 4 AC 1150 A4 4 Vacuum melting - ingotcasting 100 — — — 1250 A5 5 Converter - continuous casting 200 — — —1200 A6 6 Converter - continuous casting 250 1250 6 AC 1050 A7 7 Vacuummelting - ingot casting 120 — — — 1150 A8 8 Vacuum melting - ingotcasting 120 1300 4 AC 1050 A9 9 Vacuum melting - ingot casting 120 — — —1250 A10 10 Vacuum melting - ingot casting 120 1250 10 FC 1000 A11 11Converter - continuous casting 280 1250 10 AC 1250 A12 12 Vacuummelting - ingot casting 150 — — — 1270 A13 13 Vacuum melting - ingotcasting 120 1300 2 FC 1200 A14 14 Vacuum melting - ingot casting 150 — —— 1250 A15 15 Converter - continuous casting 200 — — — 1200 A16 16Converter - continuous casting 200 — — — 1200 A17 17 Converter -continuous casting 200 — — — 1200 A18 1 Converter - continuous casting200 — — — 1200 A19 2 Converter - continuous casting 250 1300 6 AC 1250A20 6 Converter - continuous casting 250 1350 4 AC 1050 A21 11Converter - continuous casting 280 — — — 1250 A22 15 Converter -continuous casting 200 1250 10 AC 1200 A23 16 Converter - continuouscasting 200 — — — 1200 A24 17 Converter - continuous casting 200 1200 24AC 1200 A25 1 Converter - continuous casting 200 — — — 1200 A26 6Converter - continuous casting 250 1250 5 AC 1150 Comparative B1 31Converter - continuous casting 200 — — — 1250 sample B2 32 Converter -continuous casting 200 — — — 1250 B3 33 Vacuum melting - ingot casting100 — — — 1200 B4 34 Converter - continuous casting 200 — — — 1250 B5 35Converter - continuous casting 200 — — — 1250 B6 36 Converter -continuous casting 250 1250 4 AC 1150 B7 37 Converter - continuouscasting 200 — — — 1250 B8 38 Converter - continuous casting 150 — — —1200 B9 39 Converter - continuous casting 250 1200 10 AC 1100 Hotrolling conditions Steel Rolling Cumulative plate Slab commencementRolling end reduction Steel plate Ar₃ (° C.) Classification no. no.temperature (° C.) temperature (° C.) ratio (%) thickness (mm) (Note 3)Invention A1 1 1050 980 88 25 775 sample A2 2 1120 940 84 40 750 A3 3980 870 88 25 760 A4 4 1150 1000 75 25 775 A5 5 1130 920 75 50 750 A6 6900 800 84 40 760 A7 7 1000 850 83 20 750 A8 8 970 890 58 50 750 A9 91130 1000 83 220 775 A10 10 930 860 75 30 780 A11 11 1150 1030 91 25 770A12 12 1120 860 87 20 785 A13 13 1090 850 79 25 770 A14 14 1100 880 9015 765 A15 15 1060 910 90 20 750 A16 16 1050 890 90 20 770 A17 17 1050900 90 20 770 A18 1 1050 980 88 25 775 A19 2 1130 950 84 40 750 A20 6900 820 84 40 760 A21 11 1120 1000 91 25 770 A22 15 1070 900 90 20 750A23 16 1050 890 90 20 770 A24 17 1040 880 90 20 770 A25 1 1070 990 75 50780 A26 6 980 890 84 40 765 Comparative B1 31 1130 930 88 25 780 sampleB2 32 1120 930 88 25 745 B3 33 1030 900 75 25 760 B4 34 1150 950 88 25770 B5 35 1130 900 88 25 760 B6 36 1060 850 92 20 770 B7 37 1150 930 8825 780 B8 38 1090 900 83 25 760 B9 39 980 860 90 25 715 Acceleratedcooling conditions (Note 4) Cooling Accelerated rate after Normalizingconditions (Note 6) cooling Accelerated Accelerated accelerated Ac₃transformation Cooling Steel commencement cooling end cooling coolingtemperature Heating rate plate Slab tempurature temperature rate (°C./sec.) (° C.) temperature (° C./sec.) Classification no. no. (° C.) (°C.) (° C./s) (Note 5) (Note 7) (° C.) (Note 6) Invention A1 1 850 450 250.11 — — — sample A2 2 800 350 15 0.10 — — — A3 3 780 500 25 0.13 — — —A4 4 — — — — 880 950 1.0 A5 5 850 450 10 0.15 865 930 0.5 A6 6 — — — —870 950 0.5 A7 7 — — — — 870 900 1.0 A8 8 820 350 10 0.15 — — A9 9 — — —— 890 550 0.8 A10 10 810 300 20 0.12 395 930 0.6 A11 11 900 500 25 0.22— — — A12 12 800 300 30 0.26 — — — A13 13 — — — — 380 910 1.0 A14 14 810500 30 0.33 — — — A15 15 820 450 25 0.20 — — — A16 16 830 450 25 0.20 —— — A17 17 820 450 25 0.20 — — — A18 1 850 450 25 0.11 860 900 0.6 A19 2820 350 15 0.10 — — — A20 6 — — — — 870 950 0.5 A21 11 890 500 25 0.22 —— — A22 15 820 450 25 0.20 — — — A23 16 830 450 25 0.20 865 890 1.0 A2417 830 450 25 0.20 865 930 1.0 A25 1 — — — — 860 930 0.3 A26 6 820 35015 0.10 — — — Comparative B1 31 860 450 25 0.11 — — — sample B2 32 850450 25 0.11 — — — B3 33 830 400 25 0.11 — — — B4 34 860 500 25 0.12 — —— B5 35 850 450 25 0.11 — — — D6 36 300 500 30 0.18 — — — B7 31 — — — —875 890 0.8 B8 38 850 450 25 0.11 — — — B9 39 830 450 25 0.11 — — — Areapercentage of Mn micro-segregation (%) Tempering (Note 10) Solute Mo, WSteel temperature 1 mm Solute Solute Solute plate Slab (° C.) from 1/4Thickness Mo W Mo + W Classification no. no. (Note 9) surface thicknesscenter (%) (%) (%) Invention A1 1 8 9 13 0.021 — 0.021 sample A2 2 40010 12 18 0.026 — 0.026 A3 3 — 5 4 7 0.039 — 0.039 A4 4 — 7 8 8 0.030 —0.030 A5 5 — 11 13 20 — 0.031 0.031 A6 6 — 6 8 10 0.012 0.025 0.037 A7 7450 7 6 7 0.028 0.055 0.083 A8 8 — 3 5 5 0.024 — 0.024 A9 9 — 7 7 80.022 — 0.022 A10 10 400 2 4 3 — 0.019 0.019 A11 11 — 6 8 9 0.023 —0.023 A12 12 — 5 9 9 0.026 — 0.026 A13 13 — 5 6 6 0.010 — 0.010 A14 14 —9 8 10 0.028 — 0.028 A15 15 — 6 8 11 0.028 — 0.029 A16 16 — 5 7 10 0.014— 0.014 A17 17 — 7 9 10 0.021 — 0.021 A18 1 — 8 3 12 0.015 — 0.015 A19 2400 4 5 6 0.024 — 0 024 A20 6 — 4 4 7 0.013 0.019 0.032 A21 11 — 8 9 150.024 — 0.024 A22 15 — 5 7 8 0.030 — 0.030 A23 16 — 5 7 10 0.012 — 0.012A24 17 — 4 5 8 0.018 — 0.018 A25 1 — 9 10 15 0.002 — 0.002 A26 6 600 6 79 0.002 0.002 0.004 Comparative B1 31 — 14 15 19 — — 0.000 sample B2 32— 15 15 22 — — 0.000 B3 33 — 10 9 10 0.010 — 0.010 B4 34 — 8 9 16 0.019— 0.019 B5 35 — 10 17 20 0.021 0.014 0.035 B6 36 — 8 8 10 0.012 — 0.012B7 37 — 10 11 15 0.025 0.018 0.043 B8 38 — 11 15 21 0.027 — 0.027 B9 39500 9 10 14 0.030 — 0.030 Note 1 In “Converter - continuous casting,” aslab may be as cast or breakdown rolled after casting. In “Vacuummelting - ingot casting,” the thickness of every slab is equal to ingotthickness. Note 2 AC: air cooling, FC: furnace cooling Note 3 Measuredvalues in hot working tests simulating history in actual rollingprocesses. Note 4 No entry means air cooling without acceleratedcooling. Note 5 Average cooling rate from accelerated cooling endtemperature to 100° C. Note 6 No entry means no normalizing. Note 7 Ac₃transformation temperature under heating condition of normalizing. Note8 Average cooling rate in the range from 700° C. to 300° C. Note 9Cooling is air cooling. No entry means no tempering. Note 10 Areapercentage of those portions where Mn concentration is 1.2 times averageMn concentration or more in measurement of steel plate area 5 mm × 5 mmby X-ray microanalyzer.

TABLE 3 Base material Maximum properties (Note 1) hardness Steel YieldTensile Charpy of weld plate Slab stress strength vTrs (Hv)Classification no. no. (MPa) (MPa) (° C.) (Note 2) Invention A1 1 480587 −32 274 sample A2 2 526 615 −48 321 A3 3 499 592 −51 267 A4 4 367498 −30 202 A5 5 402 535 −25 296 A6 6 351 499 −31 235 A7 7 385 530 −34233 A8 8 619 734 −69 230 A9 9 323 441 −47 214 A10 10 299 436 −68 190 A1111 503 584 −60 245 A12 12 517 592 −57 223 A13 13 345 467 −43 241 A14 14502 589 −66 232 A15 15 515 590 −68 301 A16 16 497 583 −70 236 A17 17 503586 −51 239 A18 1 354 496 −28 272 A19 2 521 613 −53 318 A20 6 349 489−35 230 A21 11 506 582 −49 248 A22 15 516 592 −70 297 A23 16 321 461 −38236 A24 17 328 475 −36 235 A25 1 349 495 −27 274 A26 6 503 602 −56 237Comparative B1 31 455 577 −26 265 sample B2 32 503 622 −58 322 B3 33 498616 −49 288 B4 34 618 697 −17 296 B5 35 520 613 −4 279 B6 36 519 624 −8280 B7 37 331 478 −15 270 B8 38 509 615 22 285 B9 39 726 803 13 373 Note1 Test pieces are cut out from thickness center at right angles torolling direction. Note 2 In conformity with JIS Z 3101

TABLE 4 Relative corrosion rate (Note 1) Corrosion Corrosion Steelcondition {circle around (1)} condition {circle around (2)}Classification plate no. Slab no. (Note 2) (Note 3) Invention A1 1 18.312.6 sample A2 2 19.1 13.8 A3 3 14.2 9.5 A4 4 16.8 11.9 A5 5 20.5 14.6A6 6 15.0 9.9 A7 7 14.6 10.3 A8 8 14.3 10.1 A9 9 13.7 9.2 A10 10 16.411.5 A11 11 19.2 13.8 A12 12 16.1 11.0 A13 13 15.1 12.3 A14 14 17.3 13.4A15 15 18.4 14.2 A16 16 16.4 15.2 A17 17 19.3 15.3 A18 1 15.8 14.9 A19 215.8 14.7 A20 6 17.0 15.9 A21 11 16.6 16.5 A22 15 16.3 15.3 A23 16 17.214.9 A24 17 18.1 16.8 A25 1 23.6 22.9 A26 6 24.0 23.1 Comparative B1 31100 100 sample B2 32 86.0 87.0 B3 33 90.0 92.0 B4 34 109.0 122.0 B5 3589.0 95.0 B6 36 43.0 43.0 B7 37 41.0 45.0 B8 38 94.8 106.3 B9 39 92.695.7 Note 1 Relative values when the corrosion rate of comparativesample B1 is regarded as 100. Corrosion rates of B1 Corrosion condition{circle around (1)}: 0.56 mg/cm²/h. Corrosion condition {circle around(2)}: 16.2 mg/cm²/h. Note 2 Corrosion condition {circle around (1)}: pH0.5 (1 vol. % HCl + 10 mass % NaCl, 30° C. × 24 h.) Note 3 Corrosioncondition {circle around (2)}: pH 0.2 (1 vol. % HCl + 20 mass % NaCl,30° C. × 24 h.)

TABLE 5 Relative Relative sludge Steel corrosion rate formation rateClassification plate no. Slab no. (Note 1) (Note 2) Invention A1 1 25.124.0 sample A2 2 25.6 23.5 A3 3 23.4 21.9 A4 4 23.9 21.8 A5 5 24.0 22.0A6 6 22.8 19.4 A7 7 21.7 17.7 A8 8 24.6 15.3 A9 9 25.0 15.1 A10 10 25.313.7 A11 11 25.0 23.8 A12 12 25.1 24.5 A13 13 23.0 19.6 A14 14 25.4 11.9A15 15 24.3 19.4 A16 16 24.1 17.8 A17 17 24.9 17.3 A18 1 25.3 23.4 A19 253.9 23.3 A20 6 22.9 14.4 A21 11 24.4 17.3 A22 15 25.3 24.3 A23 16 25.124.3 A24 17 25.1 24.3 A25 1 25.1 24.4 A26 6 32.7 24.7 Comparative B1 31100 100 sample B2 32 97.2 97.4 B3 33 98.3 100.2 B4 34 101.5 100.3 B5 35106.2 110.5 B6 36 32.7 24.6 B7 37 25.1 24.3 B8 38 24.3 24.4 B9 39 25.726.9 Note 1 Relative values when the corrosion rate (0.54 mm/y) ofcomparative sample B1 is regarded as 100. Note 2 Relative values whenthe mass of corrosion products containing precipitates of solid sulfur(1,260 mg/test piece) of comparative sample B1 is regarded as 100.

TABLE 6 Gas components CO₂ H₂S O₂ N₂ Concentration 12 vol. % 500 ppm 5vol. % Balance

INDUSTRIAL APPLICABILITY

The present invention makes it possible to provide: a steel for a weldedstructure to be used for a crude oil tank, such as an oil tank of acrude oil carrier or an aboveground or underground crude oil tank, thatexhibits excellent general and local corrosion resistance to crude oilcorrosion caused in a steel oil tank for transporting or storing crudeoil and is capable of suppressing the formation of corrosion products(sludge) containing solid sulfur; and such a crude oil tank. Therefore,the present invention contributes to the enhancement of the long-termreliability, safety, economical efficiency and so forth of a steelstructure or a marine vessel, and brings about extremely significantindustrial advantages.

1. A crude oil tank, said crude oil tank being fabricated from a steelplate having a thickness of 40 mm or less and comprising, in mass, 0.001to 0.2% C, 0.01 to 2.5% Si, 0.1 to 2% Mn, 0.03% or less P, 0.007% orless S, 0.1 to 1.5% Cu, 0.001 to 0.3% Al, 0.001 to 0.01% N, 0.03 to 01%Mo and optionally 0.01 to 0.5% W, satisfying the following expression,in mass %,Solute Mo+Solute W≧0.005%, with the balance consisting of Fe andunavoidable impurities.
 2. A crude oil tank according to claim 1,wherein the carbon equivalent (Ceq.) of the steel plate, in mass %,defined by the equation (1) is 0.4% or less;Ceq.=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+W+V)/5  (1).
 3. A crude oil tank accordingto claim 1 or 2, wherein the Cr content of the steel plate is less than0.1 mass %.
 4. A crude oil tank according to claim 1 or 2, wherein thesteel plate contains, in mass, 0.1 to 3% Ni and/or 0.1 to 3% Co.
 5. Acrude oil tank according to claim 1 or 2, wherein the steel platefurther contains, in mass, one or more of 0.01 to 0.3% Sb, 0.01 to 0.3%Sn, 0.01 to 0.3% Pb, 0.01 to 0.3% As and 0.01 to 0.3% Bi.
 6. A crude oiltank according to claim 1 or 2, wherein the steel plate furthercontains, in mass, one or more of 0.002 to 0.2% Nb, 0.005 to 0.5% V,0.002 to 0.2% Ti, 0.005 to 0.5% Ta, 0.005 to 0.5% Zr and 0.0002 to0.005% B.
 7. A crude oil tank according to claim 1 or 2, wherein thesteel plate further contains, in mass, one or more of 0.0001 to 0.01%Mg, 0.0005 to 0.01% Ca, 0.0001 to 0.1% Y, 0.005 to 0.1% La and 0.005 to0.1% Ce.
 8. A crude oil tank according to claim 1, wherein the areapercentage of microscopic segregation portions where the Mnconcentration is 1.2 times or more the average Mn concentration in thesteel plate is 10% or less.
 9. A crude oil tank according to claim 3,characterized by the steel plate further containing, in mass, 0.1 to 3%Ni and/or 0.1 to 3% Co.
 10. A crude oil tank according to claim 3,characterized by the steel plate further containing, in mass, one ormore of 0.01 to 0.3% Sb, 0.01 to 0.3% Sn, 0.01 to 0.3% Pb, 0.01 to 0.3%As and 0.01 to 0.3% Bi.
 11. A crude oil tank according to claim 4,characterized by the steel plate further containing, in mass, one ormore of 0.01 to 0.3% Sb, 0.01 to 0.3% Sn, 0.01 to 0.3% Pb, 0.01 to 0.3%As and 0.01 to 0.3% Bi.
 12. A crude oil tank according to claim 3,characterized by the steel plate further containing, in mass, one ormore of 0.002 to 0.2% Nb, 0.005 to 0.5% V, 0.002 to 0.2% Ti, 0.005 to0.5% Ta, 0.005 to 0.5% Zr and 0.0002 to 0.005% B.
 13. A crude oil tankaccording to claim 4, characterized by the steel plate furthercontaining, in mass, one or more of 0.002 to 0.2% Nb, 0.005 to 0.5% V,0.002 to 0.2% Ti, 0.005 to 0.5% Ta, 0.005 to 0.5% Zr and 0.0002 to0.005% B.
 14. A crude oil tank according to claim 5, characterized bythe steel plate further containing, in mass, one or more of 0.002 to0.2% Nb, 0.005 to 0.5% V, 0.002 to 0.2% Ti, 0.005 to 0.5% Ta, 0.005 to0.5% Zr and 0.0002 to 0.005% B.
 15. A crude oil tank according to claim3, characterized by the steel plate further containing, in mass, one ormore of 0.0001 to 0.01% Mg, 0.0005 to 0.01% Ca, 0.0001 to 0.1% Y, 0.005to 0.1% La and 0.005 to 0.1% Ce.
 16. A crude oil tank according to claim4, characterized by the steel plate further containing, in mass, one ormore of 0.0001 to 0.01% Mg, 0.0005 to 0.01% Ca, 0.0001 to 0.1% Y, 0.005to 0.1% La and 0.005 to 0.1% Ce.
 17. A crude oil tank according to claim1, characterized by the steel plate further containing, in mass, 0.03 to0.08% Mo.
 18. A crude oil tank according to claim 1, characterized bythe steel plate further containing, in mass, 0.03 to 0.07% Mo.